A tool coming of age: thapsigargin as an inhibitor of sarco-endoplasmic reticulum Ca(2+)-ATPases

Mutually exclusive teams-like patterns of gene regulation characterize phenotypic heterogeneity along the noradrenergic-mesenchymal axis in neuroblastoma

Neuroblastoma is the most frequent extracranial pediatric tumor and leads to 15% of all cancer-related deaths in children. Tumor relapse and therapy resistance in neuroblastoma are driven by phenotypic plasticity and heterogeneity between noradrenergic (NOR) and mesenchymal (MES) cell states. Despite the importance of this phenotypic plasticity, the dynamics and molecular patterns associated with these bidirectional cell-state transitions remain relatively poorly understood. Here, we analyze multiple RNA-seq datasets at both bulk and single-cell resolution, to understand the association between NOR- and MES-specific factors. We observed that NOR-specific and MES-specific expression patterns are largely mutually exclusive, exhibiting a "teams-like" behavior among the genes involved, reminiscent of our earlier observations in lung cancer and melanoma. This antagonism between NOR and MES phenotypes was also associated with metabolic reprogramming and with immunotherapy targets PD-L1 and GD2 as well as with experimental perturbations driving the NOR-MES and/or MES-NOR transition. Further, these "teams-like" patterns were seen only among the NOR- and MES-specific genes, but not in housekeeping genes, possibly highlighting a hallmark of network topology enabling cancer cell plasticity.

The molecular determinants of calcium ATPase inhibition by curcuminoids

The natural product curcumin and some of its analogs are known inhibitors of the transmembrane enzyme sarco/endoplasmic reticulum calcium ATPase (SERCA). Despite their widespread use, the curcuminoids' binding site in SERCA and their relevant interactions with the enzyme remain elusive. This lack of knowledge has prevented the development of curcuminoids into valuable experimental tools or into agents of therapeutic value. We used the crystal structures of SERCA in its E1 conformation in conjunction with computational tools such as docking and surface screens to determine the most likely curcumin binding site, along with key enzyme/inhibitor interactions. Additionally, we determined the inhibitory potencies and binding affinities for a small set of curcumin analogs. The predicted curcumin binding site is a narrow cleft in the transmembrane section of SERCA, close to the transmembrane/cytosol interface. In addition to pronounced complementarity in shape and hydrophobicity profiles between curcumin and the binding pocket, several hydrogen bonds were observed that were spread over the entire curcumin scaffold, involving residues on several transmembrane helices. Docking-predicted interactions were compatible with experimental observations for inhibitory potencies and binding affinities. Based on these findings, we propose an inhibition mechanism that assumes that the presence of a curcuminoid in the binding site arrests the catalytic cycle of SERCA by preventing it from converting from the E1 to the E2 conformation. This blockage of conformational change is accomplished by a combination of steric hinderance and hydrogen-bond-based cross-linking of transmembrane helices that require flexibility throughout the catalytic cycle.

Zinc Ionophore Pyrithione Mimics CD28 Costimulatory Signal in CD3 Activated T Cells

Zinc is an essential trace element that plays a crucial role in T cell immunity. During T cell activation, zinc is not only structurally important, but zinc signals can also act as a second messenger. This research investigates zinc signals in T cell activation and their function in T helper cell 1 differentiation. For this purpose, peripheral blood mononuclear cells were activated via the T cell receptor-CD3 complex, and via CD28 as a costimulatory signal. Fast and long-term changes in intracellular zinc and calcium were monitored by flow cytometry. Further, interferon (IFN)-γ was analyzed to investigate the differentiation into T helper 1 cells. We show that fast zinc fluxes are induced via CD3. Also, the intracellular zinc concentration dramatically increases 72 h after anti-CD3 and anti-CD28 stimulation, which goes along with the high release of IFN-γ. Interestingly, we found that zinc signals can function as a costimulatory signal for T helper cell 1 differentiation when T cells are activated only via CD3. These results demonstrate the importance of zinc signaling alongside calcium signaling in T cell differentiation.

Decreased pyruvate dehydrogenase activity in Tafazzin-deficient cells is caused by dysregulation of pyruvate dehydrogenase phosphatase 1 (PDP1)

Cardiolipin (CL), the signature lipid of the mitochondrial inner membrane, is critical for maintaining optimal mitochondrial function and bioenergetics. Disruption of CL metabolism, caused by mutations in the CL remodeling enzyme TAFAZZIN, results in the life-threatening disorder Barth syndrome (BTHS). While the clinical manifestations of BTHS, such as dilated cardiomyopathy and skeletal myopathy, point to defects in mitochondrial bioenergetics, the disorder is also characterized by broad metabolic dysregulation, including abnormal levels of metabolites associated with the tricarboxylic acid (TCA) cycle. Recent studies have identified the inhibition of pyruvate dehydrogenase (PDH), the gatekeeper enzyme for TCA cycle carbon influx, as a key deficiency in various BTHS model systems. However, the molecular mechanisms linking aberrant CL remodeling, particularly the primary, direct consequence of reduced tetralinoleoyl-CL (TLCL) levels, to PDH activity deficiency are not yet understood. In the current study, we found that remodeled TLCL promotes PDH function by directly binding to and enhancing the activity of PDH phosphatase 1 (PDP1). This is supported by our findings that TLCL uniquely activates PDH in a dose-dependent manner, TLCL binds to PDP1 in vitro, TLCL-mediated PDH activation is attenuated in the presence of phosphatase inhibitor, and PDP1 activity is decreased in Tafazzin-knockout (TAZ-KO) C2C12 myoblasts. Additionally, we observed decreased mitochondrial calcium levels in TAZ-KO cells and treating TAZ-KO cells with calcium lactate (CaLac) increases mitochondrial calcium and restores PDH activity and mitochondrial oxygen consumption rate. Based on our findings, we conclude that reduced mitochondrial calcium levels and decreased binding of PDP1 to TLCL contribute to decreased PDP1 activity in TAZ-KO cells.

Side-by-side comparison of published small molecule inhibitors against thapsigargin-induced store-operated Ca2+ entry in HEK293 cells

Calcium (Ca2+) is a key second messenger in eukaryotes, with store-operated Ca2+ entry (SOCE) being the main source of Ca2+ influx into non-excitable cells. ORAI1 is a highly Ca2+-selective plasma membrane channel that encodes SOCE. It is ubiquitously expressed in mammals and has been implicated in numerous diseases, including cardiovascular disease and cancer. A number of small molecules have been identified as inhibitors of SOCE with a variety of potential therapeutic uses proposed and validated in vitro and in vivo. These encompass both nonselective Ca2+ channel inhibitors and targeted selective inhibitors of SOCE. Inhibition of SOCE can be quantified both directly and indirectly with a variety of assay setups, making an accurate comparison of the activity of different SOCE inhibitors challenging. We have used a fluorescence based Ca2+ addback assay in native HEK293 cells to generate dose-response data for many published SOCE inhibitors. We were able to directly compare potency. Most compounds were validated with only minor and expected variations in potency, but some were not. This could be due to differences in assay setup relating to the mechanism of action of the inhibitors and highlights the value of a singular approach to compare these compounds, as well as the general need for biorthogonal validation of novel bioactive compounds. The compounds observed to be the most potent against SOCE in our study were: 7-azaindole 14d (12), JPIII (17), Synta-66 (6), Pyr 3 (5), GSK5503A (8), CM4620 (14) and RO2959 (7). These represent the most promising candidates for future development of SOCE inhibitors for therapeutic use.

ATP-mediated increase in H+ efflux from retinal Müller cells of the axolotl

Previous work has shown that activation of tiger salamander retinal radial glial cells by extracellular ATP induces a pronounced extracellular acidification, which has been proposed to be a potent modulator of neurotransmitter release. This study demonstrates that low micromolar concentrations of extracellular ATP similarly induce significant H+ effluxes from Müller cells isolated from the axolotl retina. Müller cells were enzymatically isolated from axolotl retina and H+ fluxes were measured from individual cells using self-referencing H+-selective microelectrodes. The increased H+ efflux from axolotl Müller cells induced by extracellular ATP required activation of metabotropic purinergic receptors and was dependent upon calcium released from internal stores. We further found that the ATP-evoked increase in H+ efflux from Müller cells of both tiger salamander and axolotl were sensitive to pharmacological agents known to interrupt calmodulin and protein kinase C (PKC) activity: chlorpromazine (CLP), trifluoperazine (TFP), and W-7 (all calmodulin inhibitors) and chelerythrine, a PKC inhibitor, all attenuated ATP-elicited increases in H+ efflux. ATP-initiated H+ fluxes of axolotl Müller cells were also significantly reduced by amiloride, suggesting a significant contribution by sodium-hydrogen exchangers (NHEs). In addition, α-cyano-4-hydroxycinnamate (4-cin), a monocarboxylate transport (MCT) inhibitor, also reduced the ATP-induced increase in H+ efflux in both axolotl and tiger salamander Müller cells, and when combined with amiloride, abolished ATP-evoked increase in H+ efflux. These data suggest that axolotl Müller cells are likely to be an excellent model system to understand the cell-signaling pathways regulating H+ release from glia and the role this may play in modulating neuronal signaling.NEW & NOTEWORTHY Glial cells are a key structural part of the tripartite synapse and have been suggested to regulate synaptic transmission, but the regulatory mechanisms remain unclear. We show that extracellular ATP, a potent glial cell activator, induces H+ efflux from axolotl retinal Müller (glial) cells through a calcium-dependent pathway that is likely to involve calmodulin, PKC, Na+/H+ exchange, and monocarboxylate transport, and suggest that such H+ release may play a key role in modulating neuronal transmission.

HIV-1 replication requires optimal activation of the unfolded protein response

Several human diseases including viral infections activate the unfolded protein response (UPR) due to abnormal accumulation of unfolded/misfolded proteins. However, UPR modulation and its functional relevance in HIV-1 infection lack comprehensive elucidation. This study reveals that HIV-1 activates IRE1, PERK, and ATF6 signaling pathways of UPR. The knockdown of PERK and ATF6 reduces HIV-1 long terminal repeat (LTR)-driven gene expression, whereas the endoplasmic reticulum (ER) chaperone HSPA5 prevents proteasomal degradation of HIV-1 p24 through its chaperone activity. Interestingly, overstimulation of UPR by a chemical inducer leads to anti-HIV activity through an enhanced type-1 interferon response. Also, treatment with a chemical ER stress inhibitor reduces HIV-1 replication. These findings suggest that an optimal UPR activation is crucial for effective viral replication, as either overstimulating UPR or inhibiting ER stress leads to viral suppression.

Intracellular remodeling associated with endoplasmic reticulum stress modifies biomechanical compliance of bladder cells

Bladder cells face a challenging biophysical environment: mechanical cues originating from urine flow and regular contraction to enable the filling voiding of the organ. To ensure functional adaption, bladder cells rely on high biomechanical compliance, nevertheless aging or chronic pathological conditions can modify this plasticity. Obviously the cytoskeletal network plays an essential role, however the contribution of other, closely entangled, intracellular organelles is currently underappreciated. The endoplasmic reticulum (ER) lies at a crucial crossroads, connected to both nucleus and cytoskeleton. Yet, its role in the maintenance of cell mechanical stability is less investigated. To start exploring these aspects, T24 bladder cancer cells were treated with the ER stress inducers brefeldin A (10-40nM BFA, 24 h) and thapsigargin (0.1-100nM TG, 24 h). Without impairment of cell motility and viability, BFA and TG triggered a significant subcellular redistribution of the ER; this was associated with a rearrangement of actin cytoskeleton. Additional inhibition of actin polymerization with cytochalasin D (100nM CytD) contributed to the spread of the ER toward cell periphery, and was accompanied by an increase of cellular stiffness (Young´s modulus) in the cytoplasmic compartment. Shrinking of the ER toward the nucleus (100nM TG, 2 h) was related to an increased stiffness in the nuclear and perinuclear areas. A similar short-term response profile was observed also in normal human primary bladder fibroblasts. In sum, the ER and its subcellular rearrangement seem to contribute to the mechanical properties of bladder cells opening new perspectives in the study of the related stress signaling cascades. Video Abstract.

Mitochondria-Endoplasmic Reticulum Contact Sites Dynamics and Calcium Homeostasis Are Differentially Disrupted in PINK1-PD or PRKN-PD Neurons

BACKGROUND

It is generally believed that the pathogenesis of PINK1/parkin-related Parkinson's disease (PD) is due to a disturbance in mitochondrial quality control. However, recent studies have found that PINK1 and Parkin play a significant role in mitochondrial calcium homeostasis and are involved in the regulation of mitochondria-endoplasmic reticulum contact sites (MERCSs).

OBJECTIVE

The aim of our study was to perform an in-depth analysis of the role of MERCSs and impaired calcium homeostasis in PINK1/Parkin-linked PD.

METHODS

In our study, we used induced pluripotent stem cell-derived dopaminergic neurons from patients with PD with loss-of-function mutations in PINK1 or PRKN. We employed a split-GFP-based contact site sensor in combination with the calcium-sensitive dye Rhod-2 AM and applied Airyscan live-cell super-resolution microscopy to determine how MERCSs are involved in the regulation of mitochondrial calcium homeostasis.

RESULTS

Our results showed that thapsigargin-induced calcium stress leads to an increase of the abundance of narrow MERCSs in wild-type neurons. Intriguingly, calcium levels at the MERCSs remained stable, whereas the increased net calcium influx resulted in elevated mitochondrial calcium levels. However, PINK1-PD or PRKN-PD neurons showed an increased abundance of MERCSs at baseline, accompanied by an inability to further increase MERCSs upon thapsigargin-induced calcium stress. Consequently, calcium distribution at MERCSs and within mitochondria was disrupted.

CONCLUSIONS

Our results demonstrated how the endoplasmic reticulum and mitochondria work together to cope with calcium stress in wild-type neurons. In addition, our results suggests that PRKN deficiency affects the dynamics and composition of MERCSs differently from PINK1 deficiency, resulting in differentially affected calcium homeostasis. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

ER stress decreases exosome production through adiponectin/T-cadherin-dependent and -independent pathways

Exosomes, extracellular vesicles (EVs) produced within cells, mediate both the disposal of intracellular waste and communication with distant cells, and they are involved in a variety of disease processes. Although disease modifications of exosome cargos have been well studied, it has been poorly investigated how disease processes, such as endoplasmic reticulum (ER) stress, affect EV production. We previously reported that adiponectin, an adipocyte-secreted salutary factor, increases systemic exosome levels through T-cadherin-mediated enhancement of exosome biogenesis. In the present study, we demonstrated that adiponectin/T-cadherin-dependent EV production was susceptible to ER stress and that low-dose tunicamycin significantly reduced EV production in the presence, but not in the absence, of adiponectin. Moreover, pharmacological or genetic activation of inositol-requiring enzyme 1α, a central regulator of ER stress, downregulated T-cadherin at the mRNA and protein levels as well as attenuated EV production. In addition, adiponectin/T-cadherin-independent EV production was attenuated under ER stress conditions. Repeated administration of tunicamycin to mice decreased circulating small EVs without decreasing tissue T-cadherin expression. Mechanistically, inositol-requiring enzyme 1α activation by silencing of the X-box binding protein 1 transcription factor upregulated the canonical interferon pathway and decreased EV production. The interferon pathway, when it was activated by polyinosinic-polycytidylic acid, also significantly attenuated EV production. Thus, we concluded that ER stress decreases exosome production through adiponectin/T-cadherin-dependent and -independent pathways.

The Role of Endoplasmic Reticulum Stress Response in Liver Regeneration

Exposure to hepatotoxic chemicals is involved in liver disease-related morbidity and mortality worldwide. The liver responds to damage by triggering compensatory hepatic regeneration. Physical agent or chemical-induced liver damage disrupts hepatocyte proteostasis, including endoplasmic reticulum (ER) homeostasis. Post-liver injury ER experiences a homeostatic imbalance, followed by active ER stress response signaling. Activated ER stress response causes selective upregulation of stress response genes and downregulation of many hepatocyte genes. Acetaminophen overdose, carbon tetrachloride, acute and chronic alcohol exposure, and physical injury activate the ER stress response, but details about the cellular consequences of the ER stress response on liver regeneration remain unclear. The current data indicate that inhibiting the ER stress response after partial hepatectomy-induced liver damage promotes liver regeneration, whereas inhibiting the ER stress response after chemical-induced hepatotoxicity impairs liver regeneration. This review summarizes key findings and emphasizes the knowledge gaps in the role of ER stress in injury and regeneration.

Neuronal Store-Operated Calcium Channels

The endoplasmic reticulum (ER) is the major intracellular calcium (Ca²⁺) storage compartment in eukaryotic cells. In most instances, the mobilization of Ca²⁺ from this store is followed by a delayed and sustained uptake of Ca²⁺ through Ca²⁺-permeable channels of the cell surface named store-operated Ca²⁺ channels (SOCCs). This gives rise to a store-operated Ca²⁺ entry (SOCE) that has been thoroughly investigated in electrically non-excitable cells where it is the principal regulated Ca²⁺ entry pathway. The existence of this Ca²⁺ route in neurons has long been a matter of debate. However, a growing body of experimental evidence indicates that the recruitment of Ca²⁺ from neuronal ER Ca²⁺ stores generates a SOCE. The present review summarizes the main studies supporting the presence of a depletion-dependent Ca²⁺ entry in neurons. It also addresses the question of the molecular composition of neuronal SOCCs, their expression, pharmacological properties, as well as their physiological relevance.

How do necrotic cells expose phosphatidylserine to attract their predators—What’s unique and what’s in common with apoptotic cells

Phosphatidylserine (PS) is a lipid component of the plasma membrane. It is asymmetrically distributed to the inner leaflet in live cells. In cells undergoing apoptosis, phosphatidylserine is exposed to the outer surfaces. The exposed phosphatidylserine acts as an evolutionarily conserved “eat-me” signal that attracts neighboring engulfing cells in metazoan organisms, including the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and mammals. During apoptosis, the exposure of phosphatidylserine to the outer surface of a cell is driven by the membrane scramblases and flippases, the activities of which are regulated by caspases. Cells undergoing necrosis, a kind of cell death frequently associated with cellular injuries and morphologically distinct from apoptosis, were initially believed to allow passive exposure of phosphatidylserine through membrane rupture. Later studies revealed that necrotic cells actively expose phosphatidylserine before any rupture occurs. A recent study in C. elegans further reported that the calcium ion (Ca2+) plays an essential role in promoting the exposure of phosphatidylserine on the surfaces of necrotic cells. These findings indicate that necrotic and apoptotic cells, which die through different molecular mechanisms, use common and unique mechanisms for promoting the exposure of the same “eat me” signal. This article will review the mechanisms regulating the exposure of phosphatidylserine on the surfaces of necrotic and apoptotic cells and highlight their similarities and differences.

Kinetics of the thapsigargin-induced Ca2+ mobilisation: A quantitative analysis in the HEK-293 cell line

Thapsigargin (TG) inhibits the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) pump and, when applied acutely, it initiates a Ca2+ mobilisation that begins with the loss of Ca2+ from the endoplasmic reticulum (ER) and culminates with store-operated Ca2+ entry (SOCE) from the extracellular space. Using the popular model cell line HEK-293, we quantified TG-induced changes in cytosolic and ER Ca2+ levels using FURA-2 and the FRET-based ER Ca2+ sensor D1ER, respectively. Our analysis predicts an ER Ca2+ leak of 5–6 µM⋅s−1 for the typical basal ER Ca2+ level of 335–407 µM in HEK-293 cells. The resulting cytosolic Ca2+ transients reached peak amplitudes of 0.6–1.0 µM in the absence of external Ca2+ and were amplified by SOCE that amounted to 28–30 nM⋅s−1 in 1 mM external Ca2+. Additionally, cytosolic Ca2+ transients were shaped by a Ca2+ clearance of 10–13 nM⋅s−1. Using puromycin (PURO), which enhances the ER Ca2+ leak, we show that TG-induced cytosolic Ca2+ transients are directly related to ER Ca2+ levels and to the ER Ca2+ leak. A one-compartment model incorporating ER Ca2+ leak and cytosolic Ca2+ clearance accounted satisfactorily for the basic features of TG-induced Ca2+ transients and underpinned the rule that an increase in amplitude associated with shortening of TG-induced cytosolic Ca2+ transients most likely reflects an increase in ER Ca2+ leak.

Porcine reproductive and respiratory syndrome virus infection triggers autophagy via ER stress-induced calcium signaling to facilitate virus replication

Calcium (Ca2+), a ubiquitous second messenger, plays a crucial role in many cellular functions. Viruses often hijack Ca2+ signaling to facilitate viral processes such as entry, replication, assembly, and egress. Here, we report that infection by the swine arterivirus, porcine reproductive and respiratory syndrome virus (PRRSV), induces dysregulated Ca2+ homeostasis, subsequently activating calmodulin-dependent protein kinase-II (CaMKII) mediated autophagy, and thus fueling viral replication. Mechanically, PRRSV infection induces endoplasmic reticulum (ER) stress and forms a closed ER–plasma membrane (PM) contacts, resulting the opening of store operated calcium entry (SOCE) channel and causing the ER to take up extracellular Ca2+, which is then released into the cytoplasm by inositol trisphosphate receptor (IP3R) channel. Importantly, pharmacological inhibition of ER stress or CaMKII mediated autophagy blocks PRRSV replication. Notably, we show that PRRSV protein Nsp2 plays a dominant role in the PRRSV induced ER stress and autophagy, interacting with stromal interaction molecule 1 (STIM1) and the 78 kDa glucose-regulated protein 78 (GRP78). The interplay between PRRSV and cellular calcium signaling provides a novel potential approach to develop antivirals and therapeutics for the disease outbreaks.

Presenilins regulate synaptic plasticity in the perforant pathways of the hippocampus

Mutations in the Presenilin genes (PSEN1 and PSEN2) are the major cause of familial Alzheimer's disease (AD), highlighting the importance of Presenilin (PS) in AD pathogenesis. Previous studies of PS function in the hippocampus demonstrated that loss of PS results in the impairment of short- and long-term synaptic plasticity and neurotransmitter release at hippocampal Schaffer collateral (SC) and mossy fiber (MF) synapses. Cortical input to the hippocampus through the lateral perforant pathway (LPP) and the medial perforant pathway (MPP) is critical for normal cognitive functions and is particularly vulnerable during aging and early stages of AD. Whether PS regulates synaptic function in the perforant pathways, however, remained unknown. In the current study, we investigate PS function in the LPP and MPP by performing whole-cell and field-potential electrophysiological recordings using acute hippocampal slices from postnatal forebrain-restricted excitatory neuron-specific PS conditional double knockout (cDKO) mice. We found that paired-pulse ratio (PPR) is reduced in the LPP and MPP of PS cDKO mice. Moreover, synaptic frequency facilitation or depression in the LPP or MPP, respectively, is impaired in PS cDKO mice. Notably, depletion of intracellular Ca2+ stores by inhibition of sarcoendoplasmic reticulum Ca2+ ATPase (SERCA) minics and occludes the effects of PS inactivation, as evidenced by decreases of the evoked excitatory postsynaptic currents (EPSCs) amplitude in the LPP and MPP of control neurons but no effect on the EPSC amplitude in PS cDKO neurons, suggesting that impaired intracellular calcium homeostasis in the absence of PS may contribute to the observed deficits in synaptic transmission. While spontaneous synaptic events, such as both the frequency and the amplitude of spontaneous or miniature EPSCs, are similar between PS cDKO and control neurons, long-term potentiation (LTP) is impaired in the LPP and MPP of PS cDKO mice, accompanied with reduction of evoked NMDA receptor-mediated responses. These findings show the importance of PS in the regulation of synaptic plasticity and intracellular calcium homeostasis in the hippocampal perforant pathways.

Common Markers and Small Molecule Inhibitors in Golgi Studies

In this chapter, we provide a detailed guide for the application of commonly used small molecules to study Golgi structure and function in vitro. Furthermore, we have curated a concise, validated list of endomembrane markers typically used in downstream assays to examine the consequent effect on the Golgi via microscopy and western blot after drug treatment. This chapter will be useful for researchers beginning their foray into the field of intracellular trafficking and Golgi biology.

Characterization of TRPV4-mediated signaling pathways in an optimized human choroid plexus epithelial cell line

The objectives of these studies were twofold: 1) to characterize the human choroid plexus papilloma (HIBCPP) cell line as a model of the blood-cerebrospinal fluid barrier (BCSFB) via morphology, tightness, and polarization of transporters in choroid plexus epithelia (CPe), and 2) to utilize Ussing-style electrophysiology to elucidate signaling pathways associated with the activation of the transient receptor potential vanilloid 4 (TRPV4) channel involved in cerebrospinal fluid (CSF) secretion. RT-PCR was implemented to determine gene expression of cell fate markers, junctional complex proteins, and transporters of interest. Scanning electron microscopy and confocal three-dimensional renderings of cultures grown on permeable supports were utilized to delineate the morphology of the brush border, junctional complexes, and polarization of key transporters. Electrophysiology was used to understand and explore TRPV4-mediated signaling in the HIBCPP cell line, considering both short-circuit current (Isc) and conductance responses. HIBCPP cells grown under optimized culture conditions exhibited minimal multilayering, developed an intermediate resistance monolayer, retained differentiation properties, and expressed, and correctly localized, junctional proteins and native transporters. We found that activation of TRPV4 resulted in a robust, multiphasic change in electrogenic ion flux and increase in conductance accompanied by substantial fluid secretion. This response appears to be modulated by a number of different effectors, implicating phospholipase C (PLC), protein kinase C (PKC), and phosphoinositide 3-kinase (PI3K) in TRPV4-mediated ion flux. The HIBCPP cell line is a representative model of the human BCSFB, which can be utilized for studies of transporter function, intracellular signaling, and regulation of CSF production.

The endoplasmic reticulum membrane protein Sec62 as potential therapeutic target in SEC62 overexpressing tumors

The human SEC62 gene is located on chromosome 3q, was characterized as a tumor driver gene and is found to be overexpressed in an ever-growing number of tumors, particularly those with 3q26 amplification. Where analyzed, SEC62 overexpression was associated with poor prognosis. Sec62 protein is a membrane protein of the endoplasmic reticulum (ER) and has functions in endoplasmic reticulum protein import, endoplasmic reticulum-phagy and -in cooperation with the cytosolic protein calmodulin- the maintenance of cellular calcium homeostasis. Various human tumors show SEC62 overexpression in immunohistochemistry and corresponding cell lines confirm this phenomenon in western blots and immunofluorescence. Furthermore, these tumor cells are characterized by increased stress tolerance and migratory as well as invasive potential, three hallmarks of cancer cells. Strikingly, plasmid-driven overexpression of SEC62 in non-SEC62 overexpressing cells introduces the same three hallmarks of cancer into the transfected cells. Depletion of Sec62 from either type of SEC62 overexpressing tumor cells by treatment with SEC62-targeting siRNAs leads to reduced stress tolerance and reduced migratory as well as invasive potential. Where tested, treatment of SEC62 overexpressing tumor cells with the small molecule/calmodulin antagonist trifluoperazine (TFP) phenocopied the effect of SEC62-targeting siRNAs. Recently, first phase II clinical trials with the prodrug mipsagargin/G202, which targets cellular calcium homeostasis in prostate cells as well as neovascular tissue in various tumors were started. According to experiments with tumor cell lines, however, SEC62 overexpressing tumor cells may be less responsive or resistant against such treatment. Therefore, murine tumor models for tumor growth or metastasis were evaluated with respect to their responsiveness to treatment with a mipsagargin analog (thapsigargin), or trifluoperazine, which had previously been in clinical use for the treatment of schizophrenia, or with the combination of both drugs. So far, no additive effect of the two drugs was observed but trifluoperazine had an inhibitory effect on tumor growth and metastatic potential in the models. Here, we review the state of affairs.

Role of Endothelial Kinin B1 Receptor on the Membrane Potential of Transgenic Rat Aorta

The kinin receptors are classically involved in inflammation, pain and sepsis. The effects of the kinin B1 receptor agonist des-Arg9-bradykinin (DBK) and lipopolysaccharide (LPS) were investigated by comparing the membrane potential responses of aortic rings from transgenic rats overexpressing the kinin B1 receptor (B1R) in the endothelium (TGR(Tie2B1)) and Sprague Dawley (SD) rats. No difference in the resting membrane potential in the aorta’s smooth muscle from the transgenic and SD rats was observed. The aorta rings from SD rats hyperpolarized only to LPS but not to DBK, whereas the aorta rings from TGR(Tie2B1) responded by the administration of both drugs. DBK and LPS responses were inhibited by the B1 receptor antagonist R715 and by iberiotoxin in both cases. Thapsigargin induced a hyperpolarization in the smooth muscle of SD rats that was not reversed by R715, but was reversed by iberiotoxin and this hyperpolarization was further augmented by DBK administration. These results show that the model of overexpression of vascular B1 receptors in the TGR(Tie2B1) rats represent a good model to study the role of functional B1 receptors in the absence of any pathological stimulus. The data also show that KCa channels are the final mediators of the hyperpolarizing responses to DBK and LPS. In addition, we suggest an interaction between the B1R and TLR4, since the hyperpolarization induced by LPS could be abolished in the presence of R715.

CDNF Interacts with ER Chaperones and Requires UPR Sensors to Promote Neuronal Survival

Cerebral dopamine neurotrophic factor (CDNF) is a neurotrophic factor that has beneficial effects on dopamine neurons in both in vitro and in vivo models of Parkinson's disease (PD). CDNF was recently tested in phase I-II clinical trials for the treatment of PD, but the mechanisms underlying its neuroprotective properties are still poorly understood, although studies have suggested its role in the regulation of endoplasmic reticulum (ER) homeostasis and the unfolded protein response (UPR). The aim of this study was to investigate the mechanism of action of CDNF through analyzing the involvement of UPR signaling in its anti-apoptotic function. We used tunicamycin to induce ER stress in mice in vivo and used cultured primary neurons and found that CDNF expression is regulated by ER stress in vivo and that the involvement of UPR pathways is important for the neuroprotective function of CDNF. Moreover, we used AP-MS and BiFC to perform the first interactome screening for CDNF and report novel binding partners of CDNF. These findings allowed us to hypothesize that CDNF protects neurons from ER-stress-inducing agents by modulating UPR signaling towards cell survival outcomes.

A new fluorescent sensor mitoferrofluor indicates the presence of chelatable iron in polarized and depolarized mitochondria

Mitochondrial chelatable iron contributes to the severity of several injury processes, including ischemia/reperfusion, oxidative stress, and drug toxicity. However, methods to measure this species in living cells are lacking. To measure mitochondrial chelatable iron in living cells, here we synthesized a new fluorescent indicator, mitoferrofluor (MFF). We designed cationic MFF to accumulate electrophoretically in polarized mitochondria, where a reactive group then forms covalent adducts with mitochondrial proteins to retain MFF even after subsequent depolarization. We also show in cell-free medium that Fe²⁺ (and Cu²⁺), but not Fe³⁺, Ca²⁺, or other biologically relevant divalent cations, strongly quenched MFF fluorescence. Using confocal microscopy, we demonstrate in hepatocytes that red MFF fluorescence colocalized with the green fluorescence of the mitochondrial membrane potential (ΔΨ_m) indicator, rhodamine 123 (Rh123), indicating selective accumulation into the mitochondria. Unlike Rh123, mitochondria retained MFF after ΔΨ_m collapse. Furthermore, intracellular delivery of iron with membrane-permeant Fe³⁺/8-hydroxyquinoline (FeHQ) quenched MFF fluorescence by ∼80% in hepatocytes and other cell lines, which was substantially restored by the membrane-permeant transition metal chelator pyridoxal isonicotinoyl hydrazone. We also show FeHQ quenched the fluorescence of cytosolically coloaded calcein, another Fe²⁺ indicator, confirming that Fe³⁺ in FeHQ undergoes intracellular reduction to Fe²⁺. Finally, MFF fluorescence did not change after addition of the calcium mobilizer thapsigargin, which shows MFF is insensitive to physiologically relevant increases of mitochondrial Ca²⁺. In conclusion, the new sensor reagent MFF fluorescence is an indicator of mitochondrial chelatable Fe²⁺ in normal hepatocytes with polarized mitochondria as well as in cells undergoing loss of ΔΨ_m.

Stromal Interaction Molecule 1 Promotes the Replication of vvIBDV by Mobilizing Ca2+ in the ER

Infectious bursal disease virus (IBDV) is one of the main threats to the poultry industry worldwide. Very virulent IBDV (vvIBDV) is a fatal virus strain that causes heavy mortality in young chicken flocks. Ca²⁺ is one of the most universal and versatile signalling molecules and is involved in almost every aspect of cellular processes. Clinical examination showed that one of the characteristics of vvIBDV-infected chickens was severe metabolic disorders, and the chemical examination showed that their serum Ca²⁺ level decreased significantly. However, there are limited studies on how vvIBDV infection modulates the cellular Ca²⁺ level and the effect of Ca²⁺ level changes on vvIBDV replication. In our study, we found Ca²⁺ levels in the endoplasmic reticulum (ER) of vvIBDV-infected B cells were higher than that of mock-infected cells, and the expression level of stromal interaction molecule 1 (STIM1), an ER Ca²⁺ sensor, was significantly upregulated due to vvIBDV infection. The knock-down expression of STIM1 led to decreased Ca²⁺ level in the ER and suppressed vvIBDV replication, while the over-expressed STIM1 led to ER Ca²⁺ upregulation and promoted vvIBDV replication. We also showed that the inhibition of Ca²⁺-release-activated-Ca²⁺ (CRAC) channels could reduce vvIBDV infection by blocking Ca²⁺ from entering the ER. This study suggests a new mechanism that STIM1 promotes the replication of vvIBDV by mobilizing Ca²⁺ in the ER.

Neuronal microRNAs safeguard ER Ca2+ homeostasis and attenuate the unfolded protein response upon stress

Ca2+ is a critical mediator of neurotransmitter release, synaptic plasticity, and gene expression, but also excitotoxicity. Ca2+ signaling and homeostasis are coordinated by an intricate network of channels, pumps, and calcium-binding proteins, which must be rapidly regulated at all expression levels. Τhe role of neuronal miRNAs in regulating ryanodine receptors (RyRs) and inositol 1,4,5-triphosphate receptors (IP3Rs) was investigated to understand the underlying mechanisms that modulate ER Ca2+ release. RyRs and IP3Rs are critical in mounting and propagating cytosolic Ca2+ signals by functionally linking the ER Ca2+ content, while excessive ER Ca2+ release via these receptors is central to the pathophysiology of a wide range of neurological diseases. Herein, two brain-restricted microRNAs, miR-124-3p and miR-153-3p, were found to bind to RyR1-3 and IP3R3 3'UTRs, and suppress their expression at both the mRNA and protein level. Ca2+ imaging studies revealed that overexpression of these miRNAs reduced ER Ca2+ release upon RyR/IP3R activation, but had no effect on [Ca2+]i under resting conditions. Interestingly, treatments that cause excessive ER Ca2+ release decreased expression of these miRNAs and increased expression of their target ER Ca2+ channels, indicating interdependence of miRNAs, RyRs, and IP3Rs in Ca2+ homeostasis. Furthermore, by maintaining the ER Ca2+ content, miR-124 and miR-153 reduced cytosolic Ca2+ overload and preserved protein-folding capacity by attenuating PERK signaling. Overall, this study shows that miR-124-3p and miR-153-3p fine-tune ER Ca2+ homeostasis and alleviate ER stress responses.

Calreticulin nuclear translocalization alleviates CaM/CaMKII/CREB signaling pathway to enhance chemosensitivity in HDAC inhibitor-resistant hepatocellular carcinoma cells

Calreticulin (CRT) is located in the endoplasmic reticulum (ER), it helps proteins fold correctly inside the ER, and acts as a modulator of Ca²⁺ homeostasis. Aberrant expression of CRT is implicated in several cancer types, qualifying CRT as a potential therapeutic target. However, it remains unclear how CRT affects specific oncogenic pathways. In this study, we used histone deacetylase inhibitors (HDACis) to establish drug-resistant liver cancer cells and further analyzed the molecular mechanism of development of drug resistance in those cells. The 2D gel electrophoresis and RT-PCR data showed that CRT was downregulated in HDACis-resistant cells by comparing with HA22T parental cells. We previously elucidated the development of drug-resistance in HCC cells via activation of PP1-eIF2α pathway, but not via ER stress pathway. Here, we show that thapsigargin induced ER stress through mechanism other than ER stress downstream protein GRP78-PERK to regulate CRT expression in HDACis-R cells. Moreover, the expression level of CRT was not the main cause of apoptosis in HDACis-resistant cells. Mechanistic studies identified the apoptosis factors in the nucleus-the HDACis-mediated overexpression of CRT, CRT translocation to the cell nucleus, and reduced CaM/CaMKII/CREB pathway-that led to chemosensitivity in HDACis-R HCC cells.

Age-related endoplasmic reticulum stress represses testosterone synthesis via attenuation of the circadian clock in Leydig cells

Senile animals exhibit a high risk of elevated endoplasmic reticulum (ER) stress, attenuated circadian clock, and impaired steroidogenesis in testes. However, how these three processes are intertwined in mouse Leydig cells remains unclear. In this study, a mouse model of aging and hydrogen peroxide (H₂O₂)-induced senescent TM3 Leydig cells were used to dissect the connections among ER stress, circadian oscillators, and steroidogenesis in Leydig cells. Additionally, thapsigargin (Tg, 60 nM)/tunicamycin (Tm, 60 ng/mL)-induced ER stress were established to investigate the underlying mechanisms by which ER stress regulated testosterone synthesis via circadian clock-related signaling pathways in TM3 cells and primary Leydig cells. Elevated ER stress, attenuated circadian clock, and diminished steroidogenesis were detected in the testes of aged mice (24-month-old) and H₂O₂-induced (200 μM) senescent TM3 cells in comparison with their control groups. Tg/Tm-induced ER stress reduced the transcription of the circadian clock and steroidogenic genes in TM3 cells and LH-treated (100 ng/mL) primary Leydig cells. Furthermore, 4-phenylbutyric acid (4-PBA, 1 μM), an inhibitor of ER stress, alleviated the inhibitory effect of Tg-mediated ER stress on Per2:Luc oscillations in primary Leydig cells isolated from mPer2^Luc knock-in mice, and attenuated the repressive effect of H₂O₂-induced or Tg-mediated ER stress on the transcription of circadian clock and steroidogenic genes expression and testosterone synthesis in TM3 cells. Collectively, these data indicate that age-related ER stress represses testosterone synthesis via attenuation of the circadian clock in Leydig cells.

RuBPY decreases intracellular calcium by decreasing influx and increasing storage

RuBPY is a ruthenium complex NO donor that presents a nitrite in its moiety and has been shown to induce vasodilation in various arteries, as well as arterial pressure reduction with no changes in heart rate. Since vascular tone is highly dependent on the cytosolic calcium concentration ([Ca²⁺ ]c), the current study aimed to investigate the effects of RuBPY on the intracellular mobilization of calcium stores of rat aortic vascular smooth muscle cells. Vascular reactivity experiments were performed in isolated aortic rings that were contracted with a high concentration of KCl or phenylephrine (Phe). Moreover, primary cultured vascular smooth muscle cells were used to measure [Ca²⁺ ]c by confocal microscopy. The NO donor RuBPY decreased the [Ca²⁺ ]c and reduced KCl and Phe -induced contractile responses. The selective inhibitor of sarco-endoplasmic Ca-ATPase (SERCA) with thapsigargin impaired the effect of RuBPY on Phe -induced contractile response. RuBPY also reduced caffeine-induced contraction, and the contraction dependent on the capacitive Ca²⁺ influx. Therefore, our results suggest that NO released from RuBPY decreased [Ca²⁺ ]c by calcium influx blockade, and activation of guanylyl-cyclase-cGMP-GK pathway. These results indicate that RuBPY increases Ca²⁺ storage in the sarcoplasmic reticulum by SERCA activation, and also by capacitive Ca²⁺ influx inhibition, which is dependent on the intracellular release of nitric oxide from this compound. This article is protected by copyright. All rights reserved.

Oscillatory calcium release and sustained store-operated oscillatory calcium signaling prevents differentiation of human oligodendrocyte progenitor cells

Endogenous remyelination in demyelinating diseases such as multiple sclerosis is contingent upon the successful differentiation of oligodendrocyte progenitor cells (OPCs). Signaling via the Gα_q-coupled muscarinic receptor (M_1/3R) inhibits human OPC differentiation and impairs endogenous remyelination in experimental models. We hypothesized that calcium release following Gα_q-coupled receptor (G_qR) activation directly regulates human OPC (hOPC) cell fate. In this study, we show that specific G_qR agonists activating muscarinic and metabotropic glutamate receptors induce characteristic oscillatory calcium release in hOPCs and that these agonists similarly block hOPC maturation in vitro. Both agonists induce calcium release from endoplasmic reticulum (ER) stores and store operated calcium entry (SOCE) likely via STIM/ORAI-based channels. siRNA mediated knockdown (KD) of obligate calcium sensors STIM1 and STIM2 decreased the magnitude of muscarinic agonist induced oscillatory calcium release and attenuated SOCE in hOPCs. In addition, STIM2 expression was necessary to maintain the frequency of calcium oscillations and STIM2 KD reduced spontaneous OPC differentiation. Furthermore, STIM2 siRNA prevented the effects of muscarinic agonist treatment on OPC differentiation suggesting that SOCE is necessary for the anti-differentiative action of muscarinic receptor-dependent signaling. Finally, using a gain-of-function approach with an optogenetic STIM lentivirus, we demonstrate that independent activation of SOCE was sufficient to significantly block hOPC differentiation and this occurred in a frequency dependent manner while increasing hOPC proliferation. These findings suggest that intracellular calcium oscillations directly regulate hOPC fate and that modulation of calcium oscillation frequency may overcome inhibitory Gα_q-coupled signaling that impairs myelin repair.

Notch signaling pathway: architecture, disease, and therapeutics

The NOTCH gene was identified approximately 110 years ago. Classical studies have revealed that NOTCH signaling is an evolutionarily conserved pathway. NOTCH receptors undergo three cleavages and translocate into the nucleus to regulate the transcription of target genes. NOTCH signaling deeply participates in the development and homeostasis of multiple tissues and organs, the aberration of which results in cancerous and noncancerous diseases. However, recent studies indicate that the outcomes of NOTCH signaling are changeable and highly dependent on context. In terms of cancers, NOTCH signaling can both promote and inhibit tumor development in various types of cancer. The overall performance of NOTCH-targeted therapies in clinical trials has failed to meet expectations. Additionally, NOTCH mutation has been proposed as a predictive biomarker for immune checkpoint blockade therapy in many cancers. Collectively, the NOTCH pathway needs to be integrally assessed with new perspectives to inspire discoveries and applications. In this review, we focus on both classical and the latest findings related to NOTCH signaling to illustrate the history, architecture, regulatory mechanisms, contributions to physiological development, related diseases, and therapeutic applications of the NOTCH pathway. The contributions of NOTCH signaling to the tumor immune microenvironment and cancer immunotherapy are also highlighted. We hope this review will help not only beginners but also experts to systematically and thoroughly understand the NOTCH signaling pathway.

Targeting EDEM protects against ER stress and improves development and survival in C. elegans

EDEM-1, EDEM-2 and EDEM-3 are key players for the quality control of newly synthesized proteins in the endoplasmic reticulum (ER) by accelerating disposal and degradation of misfolded proteins through ER Associated Degradation (ERAD). Although many previous studies reported the role of individual ERAD components especially in cell-based systems, still little is known about the consequences of ERAD dysfunction under physiological and ER stress conditions in the context of a multicellular organism. Here we report the first individual and combined characterization and functional interplay of EDEM proteins in Caenorhabditis elegans using single, double, and triple mutant combinations. We found that EDEM-2 has a major role in the clearance of misfolded proteins from ER under physiological conditions, whereas EDEM-1 and EDEM-3 roles become prominent under acute ER stress. In contrast to SEL-1 loss, the loss of EDEMs in an intact organism induces only a modest ER stress under physiological conditions. In addition, chronic impairment of EDEM functioning attenuated both XBP-1 activation and up-regulation of the stress chaperone GRP78/BiP, in response to acute ER stress. We also show that pre-conditioning to EDEM loss in acute ER stress restores ER homeostasis and promotes survival by activating ER hormesis. We propose a novel role for EDEM in fine-tuning the ER stress responsiveness that affects ER homeostasis and survival.

ER stress and UPR^ER malfunctions have been implicated in the pathogenesis of neurodegeneration, metabolic and inflammatory diseases as well as tumor progression and diabetes, whereby disturbed ER homeostasis negatively influences the pathology of the disease. Under ER stress conditions, the cells either activate UPR^ER-dependent cytoprotective mechanisms when ER stress is at subtoxic levels or, in case of an excessive ER stress, the cytotoxic response stimulates cell death. Here, we used Caenorhabditis elegans to study the cellular responses to ER stress at organismal level. We show that EDEMs respond differently to ER stress stimuli, and moreover, EDEMs deficiencies activate an XBP-1 independent adaptive program to promote organism survival under acute ER stress. Corroborated with the fact that loss of EDEM-2 and EDEM-3 induces resistance to acute ER stress in an intact organism, our data implicate EDEM proteins in a broader response to ER stress than previously established, which opens a new avenue for understanding the regulation of ER stress with implications for clinical and therapeutic investigations.

Kinetic monitoring of neuronal stress response to proteostasis dysfunction

Proteostasis dysfunction and activation of the unfolded protein response (UPR) are characteristic of all major neurodegenerative diseases. Nevertheless, although the UPR and proteostasis dysfunction has been studied in great detail in model organisms like yeast and mammalian cell lines, it has not yet been examined in neurons. In this study, we applied a viral vector-mediated expression of a reporter protein based on a UPR transcription factor, ATF4, and time-lapse fluorescent microscopy to elucidate how mouse primary neurons respond to pharmacological and genetic perturbations to neuronal proteostasis. In in vitro models of endoplasmic reticulum (ER) stress and proteasome inhibition, we used the ATF4 reporter to reveal the time course of the neuronal stress response relative to neurite degeneration and asynchronous cell death. We showed how potential neurodegenerative disease co-factors, ER stress and mutant α-synuclein overexpression, impacted neuronal stress response and overall cellular health. This work therefore introduces a viral vector-based reporter that yields a quantifiable readout suitable for non-cell destructive kinetic monitoring of proteostasis dysfunction in neurons by harnessing ATF4 signaling as part of the UPR activation.

Probabilistic models of uORF-mediated ATF4 translation control

ATF4 is a key transcription factor that activates transcription of genes needed to respond to cellular stress. Although the mRNA encoding ATF4 is present at constant levels in the cell during the initial response, translation of ATF4 increases under conditions of cellular stress while the global translation rate decreases. We study two models for the control system that regulates the translation of ATF4, both based on the Vattem-Wek hypothesis. This hypothesis is based on a race to reload, following the translation of a small upstream open reading frame (uORF), the ternary complex that brings the initiator tRNA to the ribosome as the 40S subunit scans along the mRNA, encountering first a start codon for an inhibitory uORF whose reading frame overlaps the start of the ATF4 coding sequence. We develop a pair of simple, analytic, probabilistic models, one of which assumes all nucleotide triplets have identical kinetic properties, while the other recognizes the existence of triplets at which the ternary complex loads more efficiently. We also consider two different functions representing the dependence of the rate of initiation at uORF1 on the ternary complex concentration. In keeping with the theme of this Special Issue, we studied the properties of these models in a Maple document, which can easily be modified to consider different parameters, translation rate initiation functions, and so on.

Endoplasmic reticulum stress-induced release and binding of calreticulin from human ovarian cancer cells

BACKGROUND

Calreticulin (CRT) is an endoplasmic reticulum (ER) chaperone, but can appear surface bound on cancers cells, including ovarian cancers (OC). We investigated at what stage of cell viability, CRT appeared associated with surface of human OC cells. CRT on pre-apoptotic tumour cells is thought to initiate their eradication via a process termed immunogenic cell death (ICD).

METHODS

We treated OC cells with the chemotherapeutic-doxorubicin (DX) known to induce translocation of CRT to some tumour cell surfaces, with and without the ER stressor-thapsigargin (TG)-and/or an ER stress inhibitor-TUDCA. We monitored translocation/release of CRT in pre-apoptotic cells by flow cytometry, immunoblotting and ELISA. We investigated the difference in binding of FITC-CRT to pre-apoptotic, apoptotic and necrotic cells and the ability of extracellular CRT to generate immature dendritic cells from THP-1 monocytes.

RESULTS

Dx-treatment increased endogenously released CRT and extracellular FITC_CRT binding to human pre-apoptotic OC cells. DX and TG also promoted cell death in OC cells which also increased CRT release. These cellular responses were significantly inhibited by TUDCA, suggesting that ER stress is partially responsible for the changes in CRT cellular distribution. Extracellular CRT induces maturation of THP-1 towards a imDC phenotype, an important component of ICD.

CONCLUSION

Collectively, these cellular responses suggest that ER stress is partially responsible for the changes in CRT cellular distribution. ER-stress regulates in part the release and binding of CRT to human OC cells where it may play a role in ICD.

Resveratrol Treatment in Human Parkin-Mutant Fibroblasts Modulates cAMP and Calcium Homeostasis Regulating the Expression of Mitochondria-Associated Membranes Resident Proteins

Parkin plays an important role in ensuring efficient mitochondrial function and calcium homeostasis. Parkin-mutant human fibroblasts, with defective oxidative phosphorylation activity, showed high basal cAMP level likely ascribed to increased activity/expression of soluble adenylyl cyclase and/or low expression/activity of the phosphodiesterase isoform 4 and to a higher Ca²⁺ level. Overall, these findings support the existence, in parkin-mutant fibroblasts, of an abnormal Ca²⁺ and cAMP homeostasis in mitochondria. In our previous studies resveratrol treatment of parkin-mutant fibroblasts induced a partial rescue of mitochondrial functions associated with stimulation of the AMPK/SIRT1/PGC-1α pathway. In this study we provide additional evidence of the potential beneficial effects of resveratrol inducing an increase in the pre-existing high Ca²⁺ level and remodulation of the cAMP homeostasis in parkin-mutant fibroblasts. Consistently, we report in these fibroblasts higher expression of proteins implicated in the tethering of ER and mitochondrial contact sites along with their renormalization after resveratrol treatment. On this basis we hypothesize that resveratrol-mediated enhancement of the Ca²⁺ level, fine-tuned by the ER-mitochondria Ca²⁺ crosstalk, might modulate the pAMPK/AMPK pathway in parkin-mutant fibroblasts.

Role of calcium oscillations in sperm physiology

Intracellular Ca²⁺ is a key regulator of cell signaling and sperm are not the exception. Cells often use cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) oscillations as a means to decodify external and internal information. [Ca²⁺]_i oscillations faster than those usually found in other cells and correlated with flagellar beat were the first to be described in sperm in 1993 by Susan Suarez, in the boar. More than 20 years passed before similar [Ca²⁺]_i oscillations were documented in human sperm, simultaneously examining their flagellar beat in three dimensions by Corkidi et al. 2017. On the other hand, 10 years after the discovery of the fast boar [Ca²⁺]_i oscillations, slower ones triggered by compounds from the egg external envelope were found to regulate cell motility and chemotaxis in sperm from marine organisms. Today it is known that sperm display fast and slow spontaneous and agonist triggered [Ca²⁺]_i oscillations. In mammalian sperm these Ca²⁺ transients may act like a multifaceted tool that regulates fundamental functions such as motility and acrosome reaction. This review covers the main sperm species and experimental conditions where [Ca²⁺]_i oscillations have been described and discusses what is known about the transporters involved, their regulation and the physiological purpose of these oscillations. There is a lot to be learned regarding the origin, regulation and physiological relevance of these Ca²⁺ oscillations.

Synthesis, computational docking and biological evaluation of celastrol derivatives as dual inhibitors of SERCA and P-glycoprotein in cancer therapy

A series of eleven celastrol derivatives was designed, synthesized, and evaluated for their in vitro cytotoxic activities against six human cancer cell lines (A549, HepG2, HepAD38, PC3, DLD-1 Bax-Bak WT and DKO) and three human normal cells (LO₂, BEAS-2B, CCD19Lu). To our knowledge, six derivatives were the first example of dipeptide celastrol derivatives. Among them, compound 3 was the most promising derivative, as it exhibited a remarkable anti-proliferative activity and improved selectivity in liver cancer HepAD38 versus human normal hepatocytes, LO₂. Compound 6 showed higher selectivity in liver cancer cells against human normal lung fibroblasts, CCD19Lu cell line. The Ca²⁺ mobilizations of 3 and 6 were also evaluated in the presence and absence of thapsigargin to demonstrate their inhibitory effects on SERCA. Derivatives 3 and 6 were found to induce apoptosis on LO₂, HepG2 and HepAD38 cells. The potential docking poses of all synthesized celastrol dipeptides and other known inhibitors were proposed by molecular docking. Finally, 3 inhibited P-gp-mediated drug efflux with greater efficiency than inhibitor verapamil in A549 lung cancer cells. Therefore, celastrol-dipeptide derivatives are potent drug candidates for the treatment of drug-resistant cancer.

ACRBP (Sp32) is involved in priming sperm for the acrosome reaction and the binding of sperm to the zona pellucida in a porcine model

In boar sperm, we have previously shown that capacitation is associated with the appearance of the p32 tyrosine phosphoprotein complex. The principal tyrosine phosphoprotein involved in this complex is the acrosin-binding protein (ACRBP), which regulates the autoconversion of proacrosin to intermediate forms of acrosin in both boar and mouse sperm. However, the complete biological role of ACRBP has not yet been elucidated. In this study, we tested the hypothesis that tyrosine phophorylation and the presence of the ACRBP in the sperm head are largely necessary to induce capacitation, the acrosome reaction (AR) and sperm-zona pellucida (ZP) binding, all of which are necessary steps for fertilization. In vitro fertilization (IVF) was performed using matured porcine oocytes and pre-capacitated boar sperm cultured with anti-phosphotyrosine antibodies or antibodies against ACRBP. Anti-ACRBP antibodies reduced capacitation and spontaneous AR (P<0.05). Sperm-ZP binding declined in the presence of anti-phosphotyrosine or anti-ACRBP antibodies. The localisation of anti-ACRBP antibodies on the sperm head, reduced the ability of the sperm to undergo the AR in response to solubilized ZP or by inhibiting the sarco/endoplasmic reticulum Ca2+-ATPase. These results support our hypothesis that tyrosine phosphorylated proteins and ACRBP are present upon the sperm surface in order to participate in sperm-ZP binding, and that ACRBP upon the surface of the sperm head facilitates capacitation and the AR in the porcine.

Inactivation of Presenilin in inhibitory neurons results in decreased GABAergic responses and enhanced synaptic plasticity

Mutations in the Presenilin genes are the major genetic cause of Alzheimer's disease (AD). Presenilin (PS) is highly expressed in the hippocampus, which is particularly vulnerable in AD. Previous studies of PS function in the hippocampus, however, focused exclusively on excitatory neurons. Whether PS regulates inhibitory neuronal function remained unknown. In the current study, we investigate PS function in GABAergic neurons by performing whole-cell and field-potential electrophysiological recordings using acute hippocampal slices from inhibitory neuron-specific PS conditional double knockout (IN-PS cDKO) mice at 2 months of age, before the onset of age-dependent loss of interneurons. We found that the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) is reduced in hippocampal CA1 neurons of IN-PS cDKO mice, whereas the amplitude of sIPSCs is normal. Moreover, the efficacy of inhibitory neurotransmission as assessed with synaptic input/output relations for evoked mono- and di-synaptic IPSCs is markedly lowered in hippocampal CA1 neurons of IN-PS cDKO mice. Consistent with these findings, IN-PS cDKO mice display enhanced paired-pulse facilitation, frequency facilitation and long-term potentiation in the Schaffer collateral-CA1 pathway. Interestingly, depletion of intracellular Ca2+ stores by inhibition of sarcoendoplasmic reticulum Ca2+ ATPase results in a reduction of IPSC amplitude in control hippocampal neurons but not in IN-PS cDKO neurons, suggesting that impaired intracellular calcium homeostasis in the absence of PS may contribute to the deficiencies in inhibitory neurotransmission. Furthermore, the amplitude of IPSCs induced by short trains of presynaptic stimulation and paired-pulse ratio are decreased in IN-PS cDKO mice. These findings show that inactivation of PS in interneurons results in decreased GABAergic responses and enhanced synaptic plasticity in the hippocampus, providing additional evidence for the importance of PS in the regulation of synaptic plasticity and calcium homeostasis.

Calcium ions trigger the exposure of phosphatidylserine on the surface of necrotic cells

Intracellular Ca²⁺ level is under strict regulation through calcium channels and storage pools including the endoplasmic reticulum (ER). Mutations in certain ion channel subunits, which cause mis-regulated Ca²⁺ influx, induce the excitotoxic necrosis of neurons. In the nematode Caenorhabditis elegans, dominant mutations in the DEG/ENaC sodium channel subunit MEC-4 induce six mechanosensory (touch) neurons to undergo excitotoxic necrosis. These necrotic neurons are subsequently engulfed and digested by neighboring hypodermal cells. We previously reported that necrotic touch neurons actively expose phosphatidylserine (PS), an “eat-me” signal, to attract engulfing cells. However, the upstream signal that triggers PS externalization remained elusive. Here we report that a robust and transient increase of cytoplasmic Ca²⁺ level occurs prior to the exposure of PS on necrotic touch neurons. Inhibiting the release of Ca²⁺ from the ER, either pharmacologically or genetically, specifically impairs PS exposure on necrotic but not apoptotic cells. On the contrary, inhibiting the reuptake of cytoplasmic Ca²⁺ into the ER induces ectopic necrosis and PS exposure. Remarkably, PS exposure occurs independently of other necrosis events. Furthermore, unlike in mutants of DEG/ENaC channels, in dominant mutants of deg-3 and trp-4, which encode Ca²⁺ channels, PS exposure on necrotic neurons does not rely on the ER Ca²⁺ pool. Our findings indicate that high levels of cytoplasmic Ca²⁺ are necessary and sufficient for PS exposure. They further reveal two Ca²⁺-dependent, necrosis-specific pathways that promote PS exposure, a “two-step” pathway initiated by a modest influx of Ca²⁺ and further boosted by the release of Ca²⁺ from the ER, and another, ER-independent, pathway. Moreover, we found that ANOH-1, the worm homolog of mammalian phospholipid scramblase TMEM16F, is necessary for efficient PS exposure in thapsgargin-treated worms and trp-4 mutants, like in mec-4 mutants. We propose that both the ER-mediated and ER-independent Ca²⁺ pathways promote PS externalization through activating ANOH-1.

Necrosis is a type of cell death that exhibits distinct morphological features such as cell and organelle swelling. Necrotic cells expose phosphatidylserine (PS)–a type of phospholipid—on their outer surfaces. Receptor molecules on phagocytes detect PS on necrotic cells and subsequently initiate the engulfment process. As necrosis is associated with stroke, cancer, neurodegenerative diseases, and heart diseases, studying necrotic cell clearance has important medical relevance. In the model organism the nematode C. elegans, we previously identified membrane proteins that promote the exposure of PS on necrotic cell surfaces by studying neurons that are induced to undergo necrosis by dominant mutations in ion channels. Here, in C. elegans, we have discovered that the necrotic insults trigger an increase of the cytoplasmic calcium ion (Ca²⁺), which in turn promotes PS externalization on necrotic cell surfaces. Furthermore, we have identified two different mechanisms that increase cytoplasmic Ca²⁺ levels, one dependent on the Ca²⁺ contribution from the endoplasmic reticulum (ER), the other independent of the ER. The Ca²⁺ signal targets ANOH-1, a worm homolog of mammalian proteins capable of externalizing PS, for promoting PS exposure on necrotic cells. Our findings reveal novel upstream regulatory mechanisms that promote necrotic cell clearance in animals.

Targeting the integrated stress response in ophthalmology

Purpose: To summarize the Integrated Stress Response (ISR) in the context of ophthalmology, with special interest on the cornea and anterior segment. Results: The ISR is a powerful and conserved signaling pathway that allows for cells to respond to a diverse array of both intracellular and extracellular stressors. The pathway is classically responsible for coordination of the cellular response to amino acid starvation, ultraviolet light, heme dysregulation, viral infection, and unfolded protein. Under normal circumstances, it is considered pro-survival and a necessary mechanism through which protein translation is controlled. However, in cases of severe or prolonged stress the pathway can promote apoptosis, and loss of normal cellular phenotype. The activation of this pathway culminates in the global inhibition of cap-dependent protein translation and the canonical expression of the activating transcription factor 4 (ATF4). Conclusion:The eye is uniquely exposed to ISR responsive stressors due to its environmental exposure and relative isolation from the circulatory system which are necessary for its function. We will discuss how this pathway is critical for the proper function of the tissue, its role in development, as well as how targeting of the pathway could alleviate key aspects of diverse ophthalmic diseases.

The cytoprotective protein MANF promotes neuronal survival independently from its role as a GRP78 cofactor

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an endoplasmic reticulum (ER)-stress-regulated protein exhibiting cytoprotective properties through a poorly understood mechanism in various in vitro and in vivo models of neuronal and non-neuronal damage. Although initially characterized as a secreted neurotrophic factor for midbrain dopamine neurons, MANF has recently gained more interest for its intracellular role in regulating the ER homeostasis, including serving as a cofactor of the chaperone glucose-regulated protein 78 (GRP78). We aimed for a better understanding of the neuroprotective mechanisms of MANF. Here we show for the first time that MANF promotes the survival of ER-stressed neurons in vitro as a general unfolded protein response (UPR) regulator, affecting several UPR pathways simultaneously. Interestingly, MANF does not affect naïve neurons. We hypothesize that MANF regulates UPR signaling toward a mode more compatible with neuronal survival. Screening of MANF interacting proteins from two mammalian cell lines revealed a conserved interactome of 15 proteins including several ER chaperones such as GRP78, GRP170, protein disulfide isomerase family A member 1, and protein disulfide isomerase family A member 6. Further characterization confirmed previously published finding that MANF is a cofactor of GRP78 interacting with its nucleotide binding domain. Using microscale thermophoresis and nuclear magnetic resonance spectroscopy, we discovered that MANF is an ATP binding protein and that ATP blocks the MANF-GRP78 interaction. Interestingly, functional analysis of the antiapoptotic properties of MANF mutants in cultured neurons revealed divergent roles of MANF as a GRP78 cofactor and as an antiapoptotic regulator of UPR. We conclude that the co-factor type interaction with GRP78 is dispensable for the survival-promoting activity of MANF in neurons.

Resveratrol inhibits Ca2+ signals and aggregation of platelets

Background

Resveratrol has been shown to inhibit platelet aggregation. However, the mechanism for this action of resveratrol remains to be clarified. The purpose of this study was to elucidate the Ca²⁺-related mechanism for the inhibitory action of resveratrol on platelet aggregation.

Methods

Ca²⁺ entry and subsequent aggregation of human platelets induced by different stimulants including thrombin, thapsigargin, and 1-oleoyl-2-acetylglycerol (OAG) were measured by the fluorescence method and light transmittance method, respectively. Each stimulant was added to a nominally Ca²⁺-free medium containing platelets, and then CaCl₂ was added to the medium to induce Ca²⁺ influx into platelets.

Results

Thapsigargin-induced Ca²⁺ entry into platelets and subsequent platelet aggregation were significantly inhibited in the presence of resveratrol at 6.25 μM or higher concentrations, while OAG-induced Ca²⁺ entry and subsequent platelet aggregation were not affected by resveratrol at concentrations up to 50 μM. In the nominally Ca²⁺-free medium, thrombin induced a small transient increase in intracellular Ca²⁺ concentrations, which was attenuated in the presence of resveratrol at 12.5 μM or higher concentrations. Thrombin-induced Ca²⁺ entry into platelets and subsequent platelet aggregation were significantly inhibited in the presence of resveratrol at 12.5 μM or higher concentrations.

Conclusions

The results suggest that resveratrol inhibits thrombin-induced platelet aggregation through decreasing Ca²⁺ release from its stores and inhibiting store-operated Ca²⁺ influx into platelets.

Large-scale RNAi screening uncovers therapeutic targets in the parasite Schistosoma mansoni

Schistosome parasites kill 250,000 people every year. Treatment of schistosomiasis relies on the drug praziquantel. Unfortunately, a scarcity of molecular tools has hindered the discovery of new drug targets. Here, we describe a large-scale RNA interference (RNAi) screen in adult Schistosoma mansoni that examined the function of 2216 genes. We identified 261 genes with phenotypes affecting neuromuscular function, tissue integrity, stem cell maintenance, and parasite survival. Leveraging these data, we prioritized compounds with activity against the parasites and uncovered a pair of protein kinases (TAO and STK25) that cooperate to maintain muscle-specific messenger RNA transcription. Loss of either of these kinases results in paralysis and worm death in a mammalian host. These studies may help expedite therapeutic development and invigorate studies of these neglected parasites.

The Emerging Role of RHOT1/Miro1 in the Pathogenesis of Parkinson's Disease

The expected increase in prevalence of Parkinson's disease (PD) as the most common neurodegenerative movement disorder over the next years underscores the need for a better understanding of the underlying molecular pathogenesis. Here, first insights provided by genetics over the last two decades, such as dysfunction of molecular and organellar quality control, are described. The mechanisms involved relate to impaired intracellular calcium homeostasis and mitochondrial dynamics, which are tightly linked to the cross talk between the endoplasmic reticulum (ER) and mitochondria. A number of proteins related to monogenic forms of PD have been mapped to these pathways, i.e., PINK1, Parkin, LRRK2, and α-synuclein. Recently, Miro1 was identified as an important player, as several studies linked Miro1 to mitochondrial quality control by PINK1/Parkin-mediated mitophagy and mitochondrial transport. Moreover, Miro1 is an important regulator of mitochondria-ER contact sites (MERCs), where it acts as a sensor for cytosolic calcium levels. The involvement of Miro1 in the pathogenesis of PD was recently confirmed by genetic evidence based on the first PD patients with heterozygous mutations in RHOT1/Miro1. Patient-based cellular models from RHOT1/Miro1 mutation carriers showed impaired calcium homeostasis, structural alterations of MERCs, and increased mitochondrial clearance. To account for the emerging role of Miro1, we present a comprehensive overview focusing on the role of this protein in PD-related neurodegeneration and highlighting new developments in our understanding of Miro1, which provide new avenues for neuroprotective therapies for PD patients.

The effect of axotomy on firing and ultrastructure of the crayfish mechanoreceptor neurons and satellite glial cells

Neurotrauma is among main causes of human disability and death. We studied effects of axotomy on ultrastructure and neuronal activity of a simple model object - an isolated crayfish stretch receptor that consists of single mechanoreceptor neurons (MRN) enwrapped by multilayer glial envelope. After isolation, MRN regularly fired until spontaneous activity cessation. Axotomy did not change significantly MRN spike amplitude and firing rate. However, the duration of neuron activity from MRN isolation to its spontaneous cessation decreased in axotomized MRN relative to intact neuron. [Ca²⁺] in MRN axon and soma increased 3-10 min after axotomy. Ca²⁺ entry through ion channels in the axolemma accelerated axotomy-stimulated firing cessation. MRN incubation with Ca²⁺ionophore ionomycin accelerated MRN inactivation, whereas Ca²⁺-channel blocker Cd²⁺ prolonged firing. Activity duration of either intact, or axotomized MRN did not change in the presence of ryanodine or dantrolene, inhibitors of ryanodin-sensitive Ca²⁺ channels in endoplasmic reticulum. Thapsigargin, inhibitor of endoplasmic reticulum Ca²⁺-ATPase, or its activator ochratoxin were ineffective. Ultrastructural study showed that the defect in the axon transected by thin scissors is sealed by fused axolemma, glial and collagen layers. Only the 30-50 μm long segment completely lost microtubules and contained swelled mitochondria. The microtubular bundle remained undamaged at 300 μm away from the axotomy site. However, mitochondria within the 200-300 μm segment were strongly condensed and lost matrix and cristae. Glial and collagen layers exhibited greater damage. Swelling and edema of glial layers, collagen disorganization and rupture occurred within this segment. Thus, axotomy stronger damages glia/collagen envelope, axonal microtubules and mitochondria.

Blockade of Oncogenic NOTCH1 with the SERCA Inhibitor CAD204520 in T Cell Acute Lymphoblastic Leukemia

The identification of SERCA (sarco/endoplasmic reticulum calcium ATPase) as a target for modulating gain-of-function NOTCH1 mutations in Notch-dependent cancers has spurred the development of this compound class for cancer therapeutics. Despite the innate toxicity challenge associated with SERCA inhibition, we identified CAD204520, a small molecule with better drug-like properties and reduced off-target Ca2+ toxicity compared with the SERCA inhibitor thapsigargin. In this work, we describe the properties and complex structure of CAD204520 and show that CAD204520 preferentially targets mutated over wild-type NOTCH1 proteins in T cell acute lymphoblastic leukemia (T-ALL) and mantle cell lymphoma (MCL). Uniquely among SERCA inhibitors, CAD204520 suppresses NOTCH1-mutated leukemic cells in a T-ALL xenografted model without causing cardiac toxicity. This study supports the development of SERCA inhibitors for Notch-dependent cancers and extends their application to cases with isolated mutations in the PEST degradation domain of NOTCH1, such as MCL or chronic lymphocytic leukemia (CLL).

Insights into the Role of Estrogen Receptor β in Triple-Negative Breast Cancer

Estrogen receptors (ERα and ERβ) are ligand-activated transcription factors that play different roles in gene regulation and show both overlapping and specific tissue distribution patterns. ERβ, contrary to the oncogenic ERα, has been shown to act as an oncosuppressor in several instances. However, while the tumor-promoting actions of ERα are well-known, the exact role of ERβ in carcinogenesis and tumor progression is not yet fully understood. Indeed, to date, highly variable and even opposite effects have been ascribed to ERβ in cancer, including for example both proliferative and growth-inhibitory actions. Recently ERβ has been proposed as a potential target for cancer therapy, since it is expressed in a variety of breast cancers (BCs), including triple-negative ones (TNBCs). Because of the dependence of TNBCs on active cellular signaling, numerous studies have attempted to unravel the mechanism(s) behind ERβ-regulated gene expression programs but the scenario has not been fully revealed. We comprehensively reviewed the current state of knowledge concerning ERβ role in TNBC biology, focusing on the different signaling pathways and cellular processes regulated by this transcription factor, as they could be useful in identifying new diagnostic and therapeutic approaches for TNBC.

NCX1 represents an ionic Na+ sensing mechanism in macrophages

Inflammation and infection can trigger local tissue Na+ accumulation. This Na+-rich environment boosts proinflammatory activation of monocyte/macrophage-like cells (MΦs) and their antimicrobial activity. Enhanced Na+-driven MΦ function requires the osmoprotective transcription factor nuclear factor of activated T cells 5 (NFAT5), which augments nitric oxide (NO) production and contributes to increased autophagy. However, the mechanism of Na+ sensing in MΦs remained unclear. High extracellular Na+ levels (high salt [HS]) trigger a substantial Na+ influx and Ca2+ loss. Here, we show that the Na+/Ca2+ exchanger 1 (NCX1, also known as solute carrier family 8 member A1 [SLC8A1]) plays a critical role in HS-triggered Na+ influx, concomitant Ca2+ efflux, and subsequent augmented NFAT5 accumulation. Moreover, interfering with NCX1 activity impairs HS-boosted inflammatory signaling, infection-triggered autolysosome formation, and subsequent antibacterial activity. Taken together, this demonstrates that NCX1 is able to sense Na+ and is required for amplifying inflammatory and antimicrobial MΦ responses upon HS exposure. Manipulating NCX1 offers a new strategy to regulate MΦ function.

Chemical chaperones reverse early suppression of regulatory circuits during unfolded protein response in B cells from common variable immunodeficiency patients

B cells orchestrate pro-survival and pro-apoptotic inputs during unfolded protein response (UPR) to translate, fold, sort, secrete and recycle immunoglobulins. In common variable immunodeficiency (CVID) patients, activated B cells are predisposed to an overload of abnormally processed, misfolded immunoglobulins. Using highly accurate transcript measurements, we show that expression of UPR genes and immunoglobulin chains differs qualitatively and quantitatively during the first 4 h of chemically induced UPR in B cells from CVID patients and a healthy subject. We tested thapsigargin or tunicamycin as stressors and 4-phenylbutyrate, dimethyl sulfoxide and tauroursodeoxycholic acid as chemical chaperones. We found an early and robust decrease of the UPR upon endoplasmic reticulum (ER) stress in CVID patient cells compared to the healthy control consistent with the disease phenotype. The chemical chaperones increased the UPR in the CVID patient cells in response to the stressors, suggesting that misfolded immunoglobulins were stabilized. We suggest that the AMP-dependent transcription factor alpha branch of the UPR is disturbed in CVID patients, underlying the observed expression behavior.