In a comment to our recent publication, Nicholls question our results and interpretation based on theoretical arguments that reveal a profound misunderstanding of our publication.

The somatostatin (SST) receptor family controls pituitary hormone secretion, but the distribution and specific roles of these receptors on the excitability and voltage-gated calcium signaling of hormone producing pituitary cells have not been fully characterized. Here we show that the rat pituitary gland expressed Sstr1, Sstr2, Sstr3, and Sstr5 receptor genes in a cell type-specific manner: Sstr1 and Sstr2 in thyrotrophs, Sstr3 in gonadotrophs and lactotrophs, Sstr2, Sstr3, and Sstr5 in somatotrophs, and none in corticotrophs and melanotrophs. Most gonadotrophs and thyrotrophs spontaneously fired high-amplitude single action potentials, which were silenced by SST without affecting intracellular calcium concentrations. In contrast, lactotrophs and somatotrophs spontaneously fired low-amplitude plateau-bursting action potentials in conjunction with calcium transients, both of which were silenced by SST. Moreover, SST inhibited GPCR-induced voltage-gated calcium signaling and hormone secretion in all cell types expressing SST receptors, but the inhibition was more pronounced in somatotrophs. The pattern of inhibition of electrical activity and calcium signaling was consistent with both direct and indirect inhibition of voltage-gated calcium channels, the latter being driven by cell type-specific hyperpolarization. These results indicate that the action of SST in somatotrophs is enhanced by the expression of several types of SST receptors and their slow desensitization, that SST may play a role in the electrical resynchronization of gonadotrophs, thyrotrophs, and lactotrophs, and that the lack of SST receptors in corticotrophs and melanotrophs keeps them excitable and ready to responses to stress.

ORAI1 is an intrinsic component of store-operated calcium entry (SOCE) that strictly regulates Ca influx in most non-excitable cells. ORAI1 is overexpressed in a wide variety of cancers, and its signal transduction has been associated with chemotherapy resistance. There is extensive proteomic interaction of ORAI1 with other channels and effectors, resulting in various altered phenotypes. However, the transcription regulation of ORAI1 is not well understood. We have found a putative G-quadruplex (G4) motif, ORAI1-Pu, in the upstream promoter region of the gene, having regulatory functions. High-resolution 3-D NMR structure elucidation suggests that ORAI1-Pu is a stable parallel-stranded G4, having a long 8-nt loop imparting dynamics without affecting the structural stability. The protruded loop further houses an E-box motif that provides a docking site for transcription factors like Zeb1. The G4 structure was also endogenously observed using Chromatin Immunoprecipitation (ChIP) with anti-G4 antibody (BG4) in the MDA-MB-231 cell line overexpressing ORAI1. Ligand-mediated stabilization suggested that the stabilized G4 represses transcription in cancer cell line MDA-MB-231. Downregulation of transcription further led to decreased Ca entry by the SOCE pathway, as observed by live-cell Fura-2 Ca imaging.

The transient receptor potential vanilloid 4 (TRPV4) ion channel is a ubiquitously expressed Ca-permeable ion channel that controls intracellular calcium ([Ca]) homeostasis in various types of cells. The physiological roles for TRPV4 are tissue specific and the mechanisms behind this specificity remain mostly unclarified. It is noteworthy that mutations in the TRPV4 channel have been associated to a broad spectrum of congenital diseases, with most of these mutations mainly resulting in gain-of-function. Mutations have been identified in human patients showing a variety of phenotypes and symptoms, mostly related to skeletal and neuromuscular disorders. Since TRPV4 is so widely expressed throughout the body, it comes as no surprise that the literature is growing in evidence linking this protein to malfunction in systems other than the skeletal and neuromuscular. In this review, we summarize the expression patterns of TRPV4 in several tissues and highlight findings of recent studies that address critical structural and functional features of this channel, particularly focusing on its interactions and signaling pathways related to Ca entry. Moreover, we discuss the roles of TRPV4 mutations in some diseases and pinpoint some of the mechanisms underlying pathological states where TRPV4's malfunction is prominent.

The conformational change in STIM1 that communicates sensing of ER calcium-store depletion from the STIM ER-luminal domain to the STIM cytoplasmic region and ultimately to ORAI channels in the plasma membrane is broadly understood. However, the structural basis for the STIM luminal-domain dimerization that drives the conformational change has proven elusive. A recently published study has approached this question via molecular dynamics simulations. The report pinpoints STIM residues that may be part of a luminal-domain dimerization interface, and provides unexpected insight into how torsional movements of the STIM luminal domains might trigger release of the cytoplasmic SOAR/CAD domain from its resting tethers to the STIM CC1 segments.

S100A1, a calcium-binding protein, plays a crucial role in regulating Ca signaling pathways in skeletal and cardiac myocytes via interactions with the ryanodine receptor (RyR) to affect Ca release and contractile performance. Biophysical studies strongly suggest that S100A1 interacts with RyRs but have been inconclusive about both the nature of this interaction and its competition with another important calcium-binding protein, calmodulin (CaM). Thus, high-resolution cryo-EM studies of RyRs in the presence of S100A1, with or without additional CaM, were needed. The elegant work by Weninger et al. demonstrates the interaction between S100A1 and RyR1 through various experiments and confirms that S100A1 activates RyR1 at sub-micromolar Ca concentrations, increasing the open probability of RyR1 channels.

Mitochondrial Ca plays a positive role in regulating pyruvate dehydrogenase, as well as the TCA cycle enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. This regulation boosts the production of reducing equivalents that fuel the electron transport chain, ultimately driving ATP production. The Mitochondrial Calcium Uniporter (MCU) is the highly selective channel responsible for mitochondrial Ca uptake when local Ca levels reach the threshold for channel activation. In a recent study, LaMoia et al. used an innovative [C]glutamine-based metabolic flux analysis method (Q-flux) to measure in vivo hepatic metabolic fluxes in liver-specific MCU mice. Surprisingly, they observed increased flux through isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. Metabolic pathways are continuously reorganized in response to intrinsic cellular signals, as well as hormonal and nutritional inputs. Integrating metabolic flux analysis into complex systems can provide deeper insights into how metabolic adaptations occur under different conditions.

Fluctuations in mitochondrial matrix Ca plays a critical role in matching energy production to cellular demand through direct effects on oxidative phosphorylation and ATP production. Disruption in mitochondrial Ca homeostasis, particularly under pathological conditions such as ischemia or heart failure, can lead to mitochondrial dysfunction, energy deficit, and eventually death of cardiomyocytes. The primary channel regulating acute mitochondrial Ca influx is the mitochondrial Ca uniporter (mtCU), which is regulated by the mitochondrial Ca uptake (MICU) proteins that were examined here.

The IP receptor (IPR) is a ubiquitously expressed Ca-release channel located in the endoplasmic reticulum (ER). Ca signals originating from the IPR initiate or regulate a plethora of cellular events, including cell life and death processes, e.g. exaggerated Ca release from the ER to the mitochondria is a trigger for apoptosis. Recently, Cho et al. (Current Biology, 2024, DOI: 10.1016/j.cub.2024.08.057) demonstrated that in epithelial monolayers a sustained [Ca] elevation caused by the IPRs is responsible for the extrusion of adjacent apoptotic cells out of the epithelial monolayer. Interestingly, the IPRs involved are associated with the desmosomes via K-Ras-induced actin-interacting protein (KRAP). This study not only highlight a novel role of the IPR in apoptosis, but also shed a new light on how KRAP -and by extension KRAP-related proteins- contribute to the regulation of IPR activity and, more broadly, underscores the crucial role of associated proteins in determining the function of IPRs.

Endoplasmic reticulum (ER) stress is triggered upon the interference with oxidative protein folding that aims to produce fully folded, disulfide-bonded and glycosylated proteins, which are then competent to exit the ER. Many of the enzymes catalyzing this process require the binding of Ca ions, including the chaperones BiP/GRP78, calnexin and calreticulin. The induction of ER stress with a variety of drugs interferes with chaperone Ca binding, increases cytosolic Cathrough the opening of ER Ca channels, and activates store-operated Ca entry (SOCE). Posttranslational modifications (PTMs) of the ER Ca handling proteins through ER stress-dependent phosphorylation or oxidation control these mechanisms, as demonstrated in the case of the sarco/endoplasmic reticulum ATPase (SERCA), inositol 1,4,5 trisphosphate receptors (IPRs) or stromal interaction molecule 1 (STIM1). Their aim is to restore ER Ca homeostasis but also to increase Ca transfer from the ER to mitochondria during ER stress. This latter function boosts ER bioenergetics, but also triggers apoptosis if ER Ca signaling persists. ER Ca toolkit oxidative modifications upon ER stress can occur within the ER lumen or in the adjacent cytosol. Enzymes involved in this redox control include ER oxidoreductin 1 (ERO1) or the thioredoxin-family protein disulfide isomerases (PDI) and ERp57. A tight, but adaptive connection between ER Ca content, ER stress and mitochondrial readouts allows for the proper functioning of many tissues, including skeletal muscle, the liver, and the pancreas, where ER stress either maintains or compromises their function, depending on its extent and context. Upon mutation of key regulators of ER Ca signaling, diseases such as muscular defects (e.g., from mutated selenoprotein N, SEPN1/SELENON), or diabetes (e.g., from mutated PERK) are the result.

In a recent publication, Hernansanz-Agusti̒n et al. propose that a sodium gradient across the inner mitochondrial membrane, generated by a Na/H activity integral to Complex I can account for half of the mitochondrial membrane potential. This conflicts with conventional electrophysiological and chemiosmotic understanding.

The field of ferroptosis research has grown exponentially since this form of cell death was first identified over a decade ago. Ferroptosis, an iron- and ROS-dependent type of cell death, is controlled by various metabolic pathways, including but not limited to redox and calcium (Ca) homeostasis, iron fluxes, mitochondrial function and lipid metabolism. Importantly, therapy-resistant tumors are particularly susceptible to ferroptotic cell death, rendering ferroptosis a promising therapeutic strategy against numerous malignancies. Calcium signals are important regulators of both cancer progression and cell death, with recent studies indicating their involvement in ferroptosis. Cells undergoing ferroptosis are characterized by plasma membrane rupture and the formation of nanopores, which facilitate influx of ions such as Ca into the affected cells. Furthermore, mitochondrial Ca²⁺ levels have been implicated in directly influencing the cellular response to ferroptosis. Despite the remarkable progress made in the field, our understanding of the contribution of Ca signals to ferroptosis remains limited. Here, we summarize key connections between Ca²⁺ signaling and ferroptosis in cancer pathobiology and discuss their potential therapeutic significance.

Wolfram syndrome (WS) is an incurable autosomal recessive disorder originally described as a mitochondriopathy. In a recent work, Liiv and colleagues found that an impaired endoplasmic reticulum (ER)-to-mitochondria calcium shuttling underlies mitochondrial dysfunction in WS models.

TRP Vanilloid 1 (TRPV1) channel, one of the major members of the TRP family was discovered to play a critical role in pain sensation, particularly inflammatory pain, and is associated with hyperalgesia, an enhanced sensitivity to pain. A new study by Fanet al."Structural basis of TRPV1 inhibition by SAF312 and cholesterol" sheds new light on the mechanistic structural basis of TRPV1 inhibition by SAF312 (Libvatrep), a TRPV1 antagonist, currently in phase II clinical trials. They discover that the binding site of SAF312 in TRPV1 is in close vicinity and partially overlaps with the binding site of cholesterol and that removal of cholesterol interferes with the ability of SAF312 to suppress TRPV1 current.