Hax-1 identified as a two-pore channel (TPC)-binding protein

TPCs: FROM PLANT TO HUMAN

In 2005, the Arabidopsis thaliana two-pore channel TPC1 channel was identified as a vacuolar Ca2+-release channel. In 2009, three independent groups published studies on mammalian TPCs as nicotinic acid adenine dinucleotide phosphate (NAADP)-activated endolysosomal Ca2+ release channels, results that were eventually challenged by two other groups, claiming mammalian TPCs to be phosphatidylinositol-3,5-bisphosphate [PI(3,5)P2]-activated Na+ channels. By now this dispute seems to have been largely reconciled. Lipophilic small molecule agonists of TPC2, mimicking either the NAADP or the PI(3,5)P2 mode of channel activation, revealed, together with structural evidence, that TPC2 can change its selectivity for Ca2+ versus Na+ in a ligand-dependent fashion (N- vs. P-type activation). Furthermore, the NAADP-binding proteins Jupiter microtubule-associated homolog 2 protein (JPT2) and Lsm12 were discovered, corroborating the hypothesis that NAADP activation of TPCs only works in the presence of these auxiliary NAADP-binding proteins. Pathophysiologically, loss or gain of function of TPCs has effects on autophagy, exocytosis, endocytosis, and intracellular trafficking, e.g., LDL cholesterol trafficking leading to fatty liver disease or viral and bacterial toxin trafficking, corroborating the roles of TPCs in infectious diseases such as Ebola or COVID-19. Defects in the trafficking of epidermal growth factor receptor and β1-integrin suggested roles in cancer. In neurodegenerative lysosomal storage disease models, P-type activation of TPC2 was found to have beneficial effects on both in vitro and in vivo hallmarks of Niemann-Pick disease type C1, Batten disease, and mucolipidosis type IV. Here, we cover the latest on the structure, function, physiology, and pathophysiology of these channels with a focus initially on plants followed by mammalian TPCs, and we discuss their potential as drug targets, including currently available pharmacology.

Molecular functions of HAX1 during disease progress

HCLS1-associated protein X-1 (HAX1) is a newly discovered multifunctional cell regulatory protein that is widely expressed in cells and has a close relationship with multiple cellular proteins. HAX1 plays important roles in various processes, including the regulation of apoptosis, maintenance of mitochondrial membrane potential stability and calcium homeostasis, occurrence and development of diseases, post-transcriptional regulation of gene expression, and host immune response after viral infection. In this article, we have reviewed the research progress on the biological functions of HAX1, thereby laying a theoretical foundation for further exploration of its underlying mechanisms and targeted application.

Endo-Lysosomal Two-Pore Channels and Their Protein Partners

Two-pore channels are ion channels expressed on acidic organelles such as the various vesicles that constitute the endo-lysosomal system. They are permeable to Ca2+ and Na+ and activated by the second messenger NAADP as well as the phosphoinositide, PI(3,5)P2 and/or voltage. Here, we review the proteins that interact with these channels including recently identified NAADP receptors.

Targeting the two-pore channel 2 in cancer progression and metastasis

The importance of Ca²⁺ signaling, and particularly Ca²⁺ channels, in key events of cancer cell function such as proliferation, metastasis, autophagy and angiogenesis, has recently begun to be appreciated. Of particular note are two-pore channels (TPCs), a group of recently identified Ca²⁺-channels, located within the endolysosomal system. TPC2 has recently emerged as an intracellular ion channel of significant pathophysiological relevance, specifically in cancer, and interest in its role as an anti-cancer drug target has begun to be explored. Herein, an overview of the cancer-related functions of TPC2 and a discussion of its potential as a target for therapeutic intervention, including a summary of clinical trials examining the TPC2 inhibitors, naringenin, tetrandrine, and verapamil for the treatment of various cancers is provided.

Endolysosomal Ca2+ Signaling in Cancer: The Role of TPC2, From Tumorigenesis to Metastasis

Ca2+ homeostasis is dysregulated in cancer cells and affects processes such as tumorigenesis, angiogenesis, autophagy, progression, and metastasis. Emerging evidence has suggested that endolysosomal cation channels sustain several cancer hallmarks involving proliferation, metastasis, and angiogenesis. Here, we investigate the role of TPC1-2, TRPML1-3, and P2×4 in cancer, with a particular focus on the role of TPC2 in cancer development, melanoma, and other cancer types as well as its endogenous and exogenous modulators. It has become evident that TPC2 plays a role in cancer; however, the precise mechanisms underlying its exact role remain elusive. TPC2 is a potential candidate for cancer biomarkers and a druggable target for future cancer therapy.

Effect of ghost pepper on cell proliferation, apoptosis, senescence and global proteomic profile in human renal adenocarcinoma cells

Chili peppers are an important constituent of many foods and contain medicinally valuable compounds, such as capsaicin and dihydrocapsaicin. As various dietary botanicals have anticancer properties, this study was aimed to examine the effect of Ghost pepper (Bhut Jolokia), one of the hottest chili peppers in the world, on cell proliferation, apoptosis, senescence and the global proteomic profile in human renal cell adenocarcinoma in vitro. 769-P human renal adenocarcinoma cells were cultured on RPMI-1640 media supplemented with fetal bovine serum (10%) and antibiotic-antimycotic solution (1%). Treatment stock solutions were prepared in ethanol. Cell proliferation was tested with phenol red-free media with capsaicin (0-400 μM), dihydrocapsaicin (0-400 μM), capsaicin + dihydrocapsaicin (5:1), and dry Ghost peppers (0-3 g L-1) for 24, 48 and 72 h. Polycaspase and senescence associated-beta-galactosidase (SA-beta-gal) activities were tested with capsaicin (400 μM), dihydrocapsaicin (400 μM), capsaicin (400 μM) + dihydrocapsaicin (80 μM), and ghost pepper (3 g L-1) treatments. Global proteomic profile of cells in control and ghost pepper treatment (3 g L-1) was analyzed after 6 h by a shotgun proteomic approach using tandem mass spectrometry. At 24 h after treatment (24 HAT), relative to control, cell proportion with capsaicin (400 μM), dihydrocapsaicin (400 μM), capsaicin (400 μM) + dihydrocapsaicin (80 μM), and ghost pepper (3 g L-1) treatments was reduced to 36%, 18%, 33% and 20%, respectively, and further reduced at 48 and 72 HAT. All treatments triggered an early polycaspase response. SA-beta-gal activity was normal or suppressed with all treatments. About 68,220 protein isoforms were identified by shotgun proteomic approach. Among these, about 8.2% were significantly affected by ghost pepper. Ghost pepper regulated various proteins involved in intrinsic and extrinsic apoptotic pathways, Ras, Rb/E2F, p53, TGF-beta, WNT-beta catenin, and calcium induced cell death pathways. Ghost pepper also induced changes in proteins related to methylation, acetylation, genome stability, cell cycle check points, carbohydrate, protein and other metabolism and cellular mechanisms. Ghost pepper exhibited antiproliferation activity by inducing apoptosis through a complex network of proteins in human renal cell adenocarcinoma in vitro.

Association of cardiac myosin-binding protein-C with the ryanodine receptor channel - putative retrograde regulation?

The cardiac muscle ryanodine receptor-Ca²⁺ release channel (RyR2) constitutes the sarcoplasmic reticulum (SR) Ca²⁺ efflux mechanism that initiates myocyte contraction, while cardiac myosin-binding protein-C (cMyBP-C; also known as MYBPC3) mediates regulation of acto-myosin cross-bridge cycling. In this paper, we provide the first evidence for the presence of direct interaction between these two proteins, forming a RyR2-cMyBP-C complex. The C-terminus of cMyBP-C binds with the RyR2 N-terminus in mammalian cells and the interaction is not mediated by a fibronectin-like domain. Notably, we detected complex formation between both recombinant cMyBP-C and RyR2, as well as between the native proteins in cardiac tissue. Cellular Ca²⁺ dynamics in HEK293 cells is altered upon co-expression of cMyBP-C and RyR2, with lowered frequency of RyR2-mediated spontaneous Ca²⁺ oscillations, suggesting that cMyBP-C exerts a potential inhibitory effect on RyR2-dependent Ca²⁺ release. Discovery of a functional RyR2 association with cMyBP-C provides direct evidence for a putative mechanistic link between cytosolic soluble cMyBP-C and SR-mediated Ca²⁺ release, via RyR2. Importantly, this interaction may have clinical relevance to the observed cMyBP-C and RyR2 dysfunction in cardiac pathologies, such as hypertrophic cardiomyopathy.

Endoplasmic reticulum (ER) stress triggers Hax1-dependent mitochondrial apoptotic events in cardiac cells

Cardiomyocyte apoptosis is a major process in pathogenesis of a number of heart diseases, including ischemic heart diseases and cardiac failure. Ensuring survival of cardiac cells by blocking apoptotic events is an important strategy to improve cardiac function. Although the role of ER disruption in inducing apoptosis has been demonstrated, we do not yet fully understand how it influences the mitochondrial apoptotic machinery in cardiac cell models. Recent investigations have provided evidences that the prosurvival protein HCLS1-associated protein X-1 (Hax1) protein is intimately associated with the pathogenesis of heart disease, mitochondrial biology, and protection from apoptotic cell death. To study the role of Hax1 upon ER stress induction, Hax1 was overexpressed in cardiac cells subjected to ER stress, and cell death parameters as well as mitochondrial alterations were examined. Our results demonstrated that the Hax1 is significantly downregulated in cardiac cells upon ER stress induction. Moreover, overexpression of Hax1 protected from apoptotic events triggered by Tunicamycin-induced ER stress. Upon treatment with Tunicamycin, Hax1 protected from mitochondrial fission, downregulation of mitofusins 1 and 2 (MFN1 and MFN2), loss of mitochondrial membrane potential (∆Ψm), production of reactive oxygen species (ROS) and apoptotic cell death. Taken together, our results suggest that Hax1 inhibits ER stress-induced apoptosis at both the pre- and post-mitochondrial levels. These findings may offer an opportunity to develop new agents that inhibit cell death in the diseased heart.

On the structure and mechanism of two-pore channels

In eukaryotes, two-pore channels (TPC1-3) comprise a family of ion channels that regulate the conductance of Na⁺ and Ca²⁺ ions across cellular membranes. TPC1-3 form endolysosomal channels, but TPC3 can also function in the plasma membrane. TPC1/3 are voltage-gated channels, but TPC2 opens in response to binding endolysosome-specific lipid phosphatidylinositol-3,5-diphosphate (PI(3,5)P₂ ). Filoviruses, such as Ebola, exploit TPC-mediated ion release as a means of escape from the endolysosome during infection. Antagonists that block TPC1/2 channel conductance abrogate filoviral infections. TPC1/2 form complexes with the mechanistic target of rapamycin complex 1 (mTORC1) at the endolysosomal surface that couple cellular metabolic state and cytosolic nutrient concentrations to the control of membrane potential and pH. We determined the X-ray structure of TPC1 from Arabidopsis thaliana (AtTPC1) to 2.87Å resolution-one of the two first reports of a TPC channel structure. Here, we summarize these findings and the implications that the structure may have for understanding endolysosomal control mechanisms and their role in human health.

Genetic and Biochemical Approaches for In Vivo and In Vitro Assessment of Protein Oligomerization: The Ryanodine Receptor Case Study

Oligomerization is often a structural requirement for proteins to accomplish their specific cellular function. For instance, tetramerization of the ryanodine receptor (RyR) is necessary for the formation of a functional Ca²⁺ release channel pore. Here, we describe detailed protocols for the assessment of protein self-association, including yeast two-hybrid (Y2H), co-immunoprecipitation (co-IP) and chemical cross-linking assays. In the Y2H system, protein self-interaction is detected by β-galactosidase assay in yeast co-expressing GAL4 bait and target fusions of the test protein. Protein self-interaction is further assessed by co-IP using HA- and cMyc-tagged fusions of the test protein co-expressed in mammalian HEK293 cells. The precise stoichiometry of the protein homo-oligomer is examined by cross-linking and SDS-PAGE analysis following expression in HEK293 cells. Using these different but complementary techniques, we have consistently observed the self-association of the RyR N-terminal domain and demonstrated its intrinsic ability to form tetramers. These methods can be applied to protein-protein interaction and homo-oligomerization studies of other mammalian integral membrane proteins.

Two-pore channels (TPCs): Novel voltage-gated ion channels with pleiotropic functions

Two-pore channels (TPC1-3) comprise a subfamily of the eukaryotic voltage-gated ion channels (VGICs) superfamily that are mainly expressed in acidic stores in plants and animals. TPCS are widespread across the animal kingdom, with primates, mice and rats lacking TPC3, and mainly act as Ca⁺ and Na⁺ channels, although it was also suggested that they could be permeable to other ions. Nowadays, TPCs have been related to the development of different diseases, including Parkinson´s disease, obesity or myocardial ischemia. Due to this, their study has raised the interest of the scientific community to try to understand their mechanism of action in order to be able to develop an efficient drug that could regulate TPCs activity. In this review, we will provide an updated view regarding TPCs structure, function and activation, as well as their role in different pathophysiological processes.

TPC: the NAADP discovery channel?

The Ca2+-mobilizing second messenger, NAADP (nicotinic acid adenine dinucleotide phosphate), has been with us for nearly 20 years and yet we still cannot fully agree on the identity of its target Ca2+-release channel. In spite of some recent robust challenges to the idea that two-pore channels (TPCs) represent the elusive "NAADP receptor", evidence continues to accumulate that TPCs are important for NAADP-mediated responses. This article will briefly outline the background and review more recent work pertaining to the TPC story.

Two-pore channels at the intersection of endolysosomal membrane traffic

Two-pore channels (TPCs) are ancient members of the voltage-gated ion channel superfamily that localize to acidic organelles such as lysosomes. The TPC complex is the proposed target of the Ca2+-mobilizing messenger NAADP, which releases Ca2+ from these acidic Ca2+ stores. Whereas details of TPC activation and native ion permeation remain unclear, a consensus has emerged around their function in regulating endolysosomal trafficking. This role is supported by recent proteomic data showing that TPCs interact with proteins controlling membrane organization and dynamics, including Rab GTPases and components of the fusion apparatus. Regulation of TPCs by PtdIns(3,5)P2 and/or NAADP (nicotinic acid adenine dinucleotide phosphate) together with their functional and physical association with Rab proteins provides a mechanism for coupling phosphoinositide and trafficking protein cues to local ion fluxes. Therefore, TPCs work at the regulatory cross-roads of (patho)physiological cues to co-ordinate and potentially deregulate traffic flow through the endolysosomal network. This review focuses on the native role of TPCs in trafficking and their emerging contributions to endolysosomal trafficking dysfunction.

A primer of NAADP-mediated Ca(2+) signalling: From sea urchin eggs to mammalian cells

Since the discovery of the Ca(2+) mobilizing effects of the pyridine nucleotide metabolite, nicotinic acid adenine dinucleotide phosphate (NAADP), this molecule has been demonstrated to function as a Ca(2+) mobilizing intracellular messenger in a wide range of cell types. In this review, I will briefly summarize the distinct principles behind NAADP-mediated Ca(2+) signalling before going on to outline the role of this messenger in the physiology of specific cell types. Central to the discussion here is the finding that NAADP principally mobilizes Ca(2+) from acidic organelles such as lysosomes and it is this property that allows NAADP to play a unique role in intracellular Ca(2+) signalling. Lysosomes and related organelles are small Ca(2+) stores but importantly may also initiate a two-way dialogue with other Ca(2+) storage organelles to amplify Ca(2+) release, and may be strategically localized to influence localized Ca(2+) signalling microdomains. The study of NAADP signalling has created a new and fruitful focus on the lysosome and endolysosomal system as major players in calcium signalling and pathophysiology.

The Two-pore channel (TPC) interactome unmasks isoform-specific roles for TPCs in endolysosomal morphology and cell pigmentation

The two-pore channels (TPC1 and TPC2) belong to an ancient family of intracellular ion channels expressed in the endolysosomal system. Little is known about how regulatory inputs converge to modulate TPC activity, and proposed activation mechanisms are controversial. Here, we compiled a proteomic characterization of the human TPC interactome, which revealed that TPCs complex with many proteins involved in Ca(2+) homeostasis, trafficking, and membrane organization. Among these interactors, TPCs were resolved to scaffold Rab GTPases and regulate endomembrane dynamics in an isoform-specific manner. TPC2, but not TPC1, caused a proliferation of endolysosomal structures, dysregulating intracellular trafficking, and cellular pigmentation. These outcomes required both TPC2 and Rab activity, as well as their interactivity, because TPC2 mutants that were inactive, or rerouted away from their endogenous expression locale, or deficient in Rab binding, failed to replicate these outcomes. Nicotinic acid adenine dinucleotide phosphate (NAADP)-evoked Ca(2+) release was also impaired using either a Rab binding-defective TPC2 mutant or a Rab inhibitor. These data suggest a fundamental role for the ancient TPC complex in trafficking that holds relevance for lysosomal proliferative scenarios observed in disease.

Ca(2+) signals, NAADP and two-pore channels: role in cellular differentiation

Ca(2+) signals regulate a wide range of physiological processes. Intracellular Ca(2+) stores can be mobilized in response to extracellular stimuli via a range of signal transduction mechanisms, often involving recruitment of diffusible second messenger molecules. The Ca(2+) -mobilizing messengers InsP3 and cADPR release Ca(2+) from the endoplasmic reticulum via the InsP3 and ryanodine receptors, respectively, while a third messenger, NAADP, releases Ca(2+) from acidic endosomes and lysosomes. Bidirectional communication between the endoplasmic reticulum (ER) and acidic organelles may have functional relevance for endolysosomal function as well as for the generation of Ca(2+) signals. The two-pore channels (TPCs) are currently strong candidates for being key components of NAADP-regulated Ca(2+) channels. Ca(2+) signals have been shown to play important roles in differentiation; however, much remains to be established about the exact signalling mechanisms involved. The investigation of the role of NAADP and TPCs in differentiation is still at an early stage, but recent studies have suggested that they are important mediators of differentiation of neurones, skeletal muscle cells and osteoclasts. NAADP signals and TPCs have also been implicated in autophagy, an important process in differentiation. Further studies will be required to identify the precise mechanism of TPC action and their link with NAADP signalling, as well as relating this to their roles in differentiation and other key processes in the cell and organism.

Competition through dimerization between antiapoptotic and proapoptotic HS-1-associated protein X-1 (Hax-1)

Studies on Hax-1 have mainly focused on variant (v) 1, demonstrating its antiapoptotic properties. However, HAX1 is heavily spliced, generating structurally distinct isoforms. We sought to characterize the Hax-1 isoforms expressed in rat heart before and after insult. We confirmed the presence of at least four Hax-1 transcripts in healthy rat cardiac muscle. These exhibited differential expression before and after induction of myocardial infarction, with v2 being up-regulated 12-fold at the transcript level and 1.5-fold at the protein level post-insult. Contrary to antiapoptotic rat and human v1, overexpression of rat v2 or human v4 (the human homologue of rat v2) in epithelial cells exacerbated cell death by 30% following H2O2 treatment compared with control vector. Coexpression of rat v1 and v2 or human v1 and v4 neutralized the protective effects of rat and human v1 and the proapoptotic effects of rat v2 and human v4 by modulating cytochrome c release. This is, at least partly, mediated by the ability of Hax-1 proteins to form homotypic and heterotypic dimers with binding affinities ranging from ~3.8 nm for v1 dimers to ~97 nm for v1/v2 dimers. The minimal binding region supporting these interactions lies between amino acids 97-278, which are shared by nearly all Hax-1 proteins, indicating that additional factors regulate the preferential formation of Hax-1 homo- or heterodimers. Our studies are the first to show that Hax-1 is a family of anti- and proapoptotic regulators that may modulate cell survival and death through homo- or heterodimerization.