Citrinin (CTN) is a mycotoxin found in crops and agricultural products and poses a serious threat to human and animal health. The aim of this study is to investigate the hepatotoxicity of CTN in mice and analyze its mechanisms from Ca2+-dependent endoplasmic reticulum (ER) stress perspective. We showed that CTN induced histopathological damage, caused ultrastructural changes in liver cells, and induced abnormal values of biochemical laboratory tests of some liver functions in mice. Treatment with CTN could induce nitric oxide (NO), malondialdehyde (MDA), and reactive oxygen species (ROS) accumulation in mice, accompanied with losses of activities of superoxide dismutase (SOD) and catalase (CAT), levels of glutathione (GSH), and capacities of total antioxidant (T-AOC), resulting in oxidative stress in mice. Furthermore, CTN treatment significantly increased Ca2+ accumulation, upregulated protein expressions of ER stress-mediated apoptosis signal protein (glucose regulated protein 78 (GRP78/BIP), C/EBP-homologous protein (CHOP), Caspase-12, and Caspase-3), and induced hepatocyte apoptosis. These adverse effects were counteracted by 4-phenylbutyric acid (4-PBA), an ER stress inhibitor. In summary, our results showed a possible underlying molecular mechanism for CTN that induced hepatocyte apoptosis in mice by the regulation of the Ca2+/ER stress signaling pathway.

/pdf/citrinin-induced-hepatotoxicity-in-mice-is-regulated-by-the-eapyrun2.pdf

Citrinin-Induced Hepatotoxicity in Mice Is Regulated by the Ca2+/Endoplasmic Reticulum Stress Signaling Pathway

Control of STIM and Orai function by post-translational modifications.

http://www.jbc.org/article/S0021925822007451/pdf

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

S-Palmitoylation of calcineurin β1 connects cellular Ca2+ homeostasis to phosphatidylinositol 4-kinase activity at the plasma membrane.

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.

https://www.frontiersin.org/articles/10.3389/fphys.2022.1040968/pdf

Regulation of T cell function by protein S-acylation

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.

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

The initiating events happening during store‐operated calcium entry are not completely understood despite the tremendous importance of this process in immune responses. Upon endoplasmic reticulum (ER) calcium depletion, STIM1 and Orai1 form calcium release‐activated calcium (CRAC) channels to allow calcium influx from the extracellular milieu. S‐acylation is a reversible post‐translational modification of cysteine residues that affects protein structure, stability, and targeting to subdomains in the plasma membrane. Previously, we have shown that Orai1 is rapidly S‐acylated following T cell activation in Jurkat cells as well as ER calcium store depletion in HEK293 cells. Here we demonstrate that STIM1 is S‐acylated at C437 residue upon T cell activation and ER calcium store depletion. This is an interesting observation because very little is known about S‐acylation and its consequences in ER‐resident proteins. Upon store depletion, STIM1 adopts an extended conformation that permits binding to Orai1. We hypothesize that S‐acylation of STIM1 promotes and stabilizes the extended conformation of STIM1. Consistent with this hypothesis, we show that substitution of cysteine 437 in the STIM1 Orai1 activating region to a serine decreases calcium entry after store depletion. We further show that the ability of STIM1 to form puncta depends on its S‐acylation at this residue. Together with S‐acylation of Orai1, these data demonstrate that CRAC channel components are rapidly S‐acylated after ER calcium store depletion. In summary, based on our previous published literature on Orai1 S‐acylation, and current discovery of S‐acylation of STIM1, it is evident that S‐acylation recruits Orai1 to specialized membrane subdomains to form puncta with STIM1, whose extended conformation is stabilized by S‐acylation at its 437 residue. Hence, S‐acylation of Orai1 and STIM1 play a crucial role facilitate store‐operated calcium entry.

Qioachu Wang

Papers

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

S‐acylation of STIM1 regulates store‐operated calcium entry