Bidirectional regulation of calcium release-activated calcium (CRAC) channel by SARAF

Orai1/STIMs modulators in pulmonary vascular diseases

Calcium (Ca2+) is a secondary messenger that regulates various cellular processes. However, Ca2+ mishandling could lead to pathological conditions. Orai1 is a Ca2+channel contributing to the store-operated calcium entry (SOCE) and plays a critical role in Ca2+ homeostasis in several cell types. Dysregulation of Orai1 contributed to severe combined immune deficiency syndrome, some cancers, pulmonary arterial hypertension (PAH), and other cardiorespiratory diseases. During its activation process, Orai1 is mainly regulated by stromal interacting molecule (STIM) proteins, especially STIM1; however, many other regulatory partners have also been recently described. Increasing knowledge about these regulatory partners provides a better view of the downstream signalling pathways of SOCE and offers an excellent opportunity to decipher Orai1 dysregulation in these diseases. These proteins participate in other cellular functions, making them attractive therapeutic targets. This review mainly focuses on Orai1 regulatory partners in the physiological and pathological conditions of the pulmonary circulation and inflammation.

Reduced Ca2+ mobilization in neonatal human platelets involves SARAF and pannexin-1

Platelets from neonates have been shown to exhibit a reduced response to physiological agonists, such as thrombin; however, the mechanism behind these findings is poorly understood. Berna-Erro et al. now provide differences in SARAF and pannexin-1 expression and function between neonatal and maternal platelets that might shed some light on the underlying mechanism. Commentary on: Berna-Erro. SARAF overexpression impairs thrombin-induced Ca2+ homeostasis in neonatal platelets. Br J Haematol 2024;204:988-1004.

Cholecalciferol Supplementation Induced Up-Regulation of SARAF Gene and Down-Regulated miR-155-5p Expression in Slovenian Patients with Multiple Sclerosis

Multiple sclerosis is a common immune-mediated inflammatory and demyelinating disease. Lower cholecalciferol levels are an established environmental risk factor in multiple sclerosis. Although cholecalciferol supplementation in multiple sclerosis is widely accepted, optimal serum levels are still debated. Moreover, how cholecalciferol affects pathogenic disease mechanisms is still unclear. In the present study, we enrolled 65 relapsing-remitting multiple sclerosis patients who were double-blindly divided into two groups with low and high cholecalciferol supplementation, respectively. In addition to clinical and environmental parameters, we obtained peripheral blood mononuclear cells to analyze DNA, RNA, and miRNA molecules. Importantly, we investigated miRNA-155-5p, a previously published pro-inflammatory miRNA in multiple sclerosis known to be correlated to cholecalciferol levels. Our results show a decrease in miR-155-5p expression after cholecalciferol supplementation in both dosage groups, consistent with previous observations. Subsequent genotyping, gene expression, and eQTL analyses reveal correlations between miR-155-5p and the SARAF gene, which plays a role in the regulation of calcium release-activated channels. As such, the present study is the first to explore and suggest that the SARAF miR-155-5p axis hypothesis might be another mechanism by which cholecalciferol supplementation might decrease miR-155 expression. This association highlights the importance of cholecalciferol supplementation in multiple sclerosis and encourages further investigation and functional cell studies.

The SOAR of STIM1 interacts with plasma membrane lipids to form ER-PM contact sites

Depletion of Ca²⁺ from the endoplasmic reticulum (ER) causes the ER Ca²⁺ sensor STIM1 to form membrane contact sites (MCSs) with the plasma membrane (PM). At the ER-PM MCS, STIM1 binds to Orai channels to induce cellular Ca²⁺ entry. The prevailing view of this sequential process is that STIM1 interacts with the PM and with Orai1 using two separate modules: a C-terminal polybasic domain (PBD) for the interaction with PM phosphoinositides and the STIM-Orai activation region (SOAR) for the interaction with Orai channels. Here, using electron and fluorescence microscopy and protein-lipid interaction assays, we show that oligomerization of the SOAR promotes direct interaction with PM phosphoinositides to trap STIM1 at ER-PM MCSs. The interaction depends on a cluster of conserved lysine residues within the SOAR and is co-regulated by the STIM1 coil-coiled 1 and inactivation domains. Collectively, our findings uncover a molecular mechanism for formation and regulation of ER-PM MCSs by STIM1.

The Cytoplasmic Region of SARAF Reduces Triple-Negative Breast Cancer Metastasis through the Regulation of Store-Operated Calcium Entry

Triple-negative breast cancer has a poor prognosis and is non-responsive to first-line therapies; hence, new therapeutic strategies are needed. Enhanced store-operated Ca2+ entry (SOCE) has been widely described as a contributing factor to tumorigenic behavior in several tumor types, particularly in breast cancer cells. SOCE-associated regulatory factor (SARAF) acts as an inhibitor of the SOCE response and, therefore, can be a potential antitumor factor. Herein, we generated a C-terminal SARAF fragment to evaluate the effect of overexpression of this peptide on the malignancy of triple-negative breast cancer cell lines. Using both in vitro and in vivo approaches, we showed that overexpression of the C-terminal SARAF fragment reduced proliferation, cell migration, and the invasion of murine and human breast cancer cells by decreasing the SOCE response. Our data suggest that regulating the activity of the SOCE response via SARAF activity might constitute the basis for further alternative therapeutic strategies for triple-negative breast cancer.

Store-Operated Calcium Entry in Breast Cancer Cells Is Insensitive to Orai1 and STIM1 N-Linked Glycosylation

N-linked glycosylation is a post-translational modification that affects protein function, structure, and interaction with other proteins. The store-operated Ca²⁺ entry (SOCE) core proteins, Orai1 and STIM1, exhibit N-glycosylation consensus motifs. Abnormal SOCE has been associated to a number of disorders, including cancer, and alterations in Orai1 glycosylation have been related to cancer invasiveness and metastasis. Here we show that treatment of non-tumoral breast epithelial cells with tunicamycin attenuates SOCE. Meanwhile, tunicamycin was without effect on SOCE in luminal MCF7 and triple negative breast cancer (TNBC) MDA-MB-231 cells. Ca²⁺ imaging experiments revealed that expression of the glycosylation-deficient Orai1 mutant (Orai1N223A) did not alter SOCE in MCF10A, MCF7 and MDA-MB-231 cells. However, expression of the non-glycosylable STIM1 mutant (STIM1N131/171Q) significantly attenuated SOCE in MCF10A cells but was without effect in SOCE in MCF7 and MDA-MB-231 cells. In non-tumoral cells impairment of STIM1 N-linked glycosylation attenuated thapsigargin (TG)-induced caspase-3 activation while in breast cancer cells, which exhibit a smaller caspase-3 activity in response to TG, expression of the non-glycosylable STIM1 mutant (STIM1N131/171Q) was without effect on TG-evoked caspase-3 activation. Summarizing, STIM1 N-linked glycosylation is essential for full SOCE activation in non-tumoral breast epithelial cells; by contrast, SOCE in breast cancer MCF7 and MDA-MB-231 cells is insensitive to Orai1 and STIM1 N-linked glycosylation, and this event might participate in the development of apoptosis resistance.

Recent Developments on the Roles of Calcium Signals and Potential Therapy Targets in Cervical Cancer

Intracellular calcium (Ca²⁺) concentration ([Ca²⁺]i) is implicated in proliferation, invasion, and metastasis in cancerous tissues. A variety of oncologic therapies and some candidate drugs induce their antitumor effects (in part or in whole) through the modulation of [Ca²⁺]i. Cervical cancer is one of most common cancers among women worldwide. Recently, major research advances relating to the Ca²⁺ signals in cervical cancer are emerging. In this review, we comprehensively describe the current progress concerning the roles of Ca²⁺ signals in the occurrence, development, and prognosis of cervical cancer. It will enhance our understanding of the causative mechanism of Ca²⁺ signals in cervical cancer and thus provide new sights for identifying potential therapeutic targets for drug discovery.

Mutations in proteins involved in E-C coupling and SOCE and congenital myopathies

In skeletal muscle, Ca²⁺ necessary for muscle contraction is stored and released from the sarcoplasmic reticulum (SR), a specialized form of endoplasmic reticulum through the mechanism known as excitation–contraction (E-C) coupling. Following activation of skeletal muscle contraction by the E-C coupling mechanism, replenishment of intracellular stores requires reuptake of cytosolic Ca²⁺ into the SR by the activity of SR Ca²⁺-ATPases, but also Ca²⁺ entry from the extracellular space, through a mechanism called store-operated calcium entry (SOCE). The fine orchestration of these processes requires several proteins, including Ca²⁺ channels, Ca²⁺ sensors, and Ca²⁺ buffers, as well as the active involvement of mitochondria. Mutations in genes coding for proteins participating in E-C coupling and SOCE are causative of several myopathies characterized by a wide spectrum of clinical phenotypes, a variety of histological features, and alterations in intracellular Ca²⁺ balance. This review summarizes current knowledge on these myopathies and discusses available knowledge on the pathogenic mechanisms of disease.

Endoplasmic Reticulum in Metaplasticity: From Information Processing to Synaptic Proteostasis

The ER (endoplasmic reticulum) is a Ca²⁺ reservoir and the unique protein-synthesizing machinery which is distributed throughout the neuron and composed of multiple different structural domains. One such domain is called EMC (endoplasmic reticulum membrane protein complex), pleiotropic nature in cellular functions. The ER/EMC position inside the neurons unmasks its contribution to synaptic plasticity via regulating various cellular processes from protein synthesis to Ca²⁺ signaling. Since presynaptic Ca²⁺ channels and postsynaptic ionotropic receptors are organized into the nanodomains, thus ER can be a crucial player in establishing TMNCs (transsynaptic molecular nanocolumns) to shape efficient neural communications. This review hypothesized that ER is not only involved in stress-mediated neurodegeneration but also axon regrowth, remyelination, neurotransmitter switching, information processing, and regulation of pre- and post-synaptic functions. Thus ER might not only be a protein-synthesizing and quality control machinery but also orchestrates plasticity of plasticity (metaplasticity) within the neuron to execute higher-order brain functions and neural repair.

Photopharmacological modulation of native CRAC channels using azoboronate photoswitches

Calcium release–activated calcium (CRAC) channels play key roles in the regulation of cellular signaling, transcription, and migration. Here, we describe the design, chemical synthesis, and characterization of photoswitchable channel inhibitors that can be switched on and off depending on the wavelength of light used. We use the compounds to induce light-dependent modulation of channel activity and downstream gene expression in human immune cells. We further expand the usage of the compounds to control seeding of cancer cells in target tissue and regulation of response to noxious stimuli in vivo in mice.

Store-operated calcium entry through calcium release–activated calcium (CRAC) channels replenishes intracellular calcium stores and plays a critical role in cellular calcium signaling. CRAC channels are activated by tightly regulated interaction between the endoplasmic reticulum (ER) calcium sensor STIM proteins and plasma membrane (PM) Orai channels. Our current understanding of the role of STIM–Orai-dependent calcium signals under physiologically relevant conditions remains limited in part due to a lack of spatiotemporally precise methods for direct manipulation of endogenous CRAC channels. Here, we report the synthesis and characterization of azoboronate light-operated CRAC channel inhibitors (LOCIs) that allow for a dynamic and fully reversible remote modulation of the function of native CRAC channels using ultraviolet (UV) and visible light. We demonstrate the use of LOCI-1 to modulate gene expression in T lymphocytes, cancer cell seeding at metastatic sites, and pain-related behavior.

CRACR2A associated immunodeficiency - the supporting factor takes center stage

Loss of function mutations in one of the core components of the calcium release-activated calcium (CRAC) channel complex, STIM1 and ORAI1, lead to severe immunodeficiency. Whether changes in regulatory components of the CRAC channel may also contribute to a disease state remained unknown. In a recent study, Wu et al. report a case of late onset immunodeficiency in a patient with bi-allelic mutation in CRACR2A, a regulatory component of the CRAC channel.