Regulated assembly of vacuolar ATPase is increased during cluster disruption-induced maturation of dendritic cells through a phosphatidylinositol 3-kinase/mTOR-dependent pathway

Human V-ATPase function is positively and negatively regulated by TLDc proteins

Proteins that contain a highly conserved TLDc domain (Tre2/Bub2/Cdc16 LysM domain catalytic) offer protection against oxidative stress and are widely implicated in neurological health and disease. How this family of proteins exerts their function, however, is poorly understood. We have recently found that the yeast TLDc protein, Oxr1p, inhibits the proton pumping vacuolar ATPase (V-ATPase) by inducing disassembly of the pump. While loss of TLDc protein function in mammals shares disease phenotypes with V-ATPase defects, whether TLDc proteins impact human V-ATPase activity directly is unclear. Here we examine the effects of five human TLDc proteins, TLDC2, NCOA7, OXR1, TBC1D24, and mEAK7 on the activity of the human V-ATPase. We find that while TLDC2, TBC1D24, and the TLDc domains of OXR1 and NCOA7 inhibit V-ATPase by inducing enzyme disassembly, mEAK7 activates the pump. The data thus shed new light both on mammalian TLDc protein function and V-ATPase regulation.

Vacuolar H+-ATPase in Diabetes, Hypertension, and Atherosclerosis

Vacuolar H+-ATPase (V-ATPase) is a multisubunit protein complex which, along with its accessory proteins, resides in almost every eukaryotic cell. It acts as a proton pump and as such is responsible for regulating pH in lysosomes, endosomes, and the extracellular space. Moreover, V-ATPase has been implicated in receptor-mediated signaling. Although numerous studies have explored the role of V-ATPase in cancer, osteoporosis, and neurodegenerative diseases, research on its involvement in vascular disease remains limited. Vascular diseases pose significant challenges to human health. This review aimed to shed light on the role of V-ATPase in hypertension and atherosclerosis. Furthermore, given that vascular complications are major complications of diabetes, this review also discusses the pathways through which V-ATPase may contribute to such complications. Beginning with an overview of the structure and function of V-ATPase in hypertension, atherosclerosis, and diabetes, this review ends by exploring the pharmacological potential of targeting V-ATPase.

Host-directed therapy for bacterial infections -Modulation of the phagolysosome pathway

Bacterial infections still impose a significant burden on humanity, even though antimicrobial agents have long since been developed. In addition to individual severe infections, the f fatality rate of sepsis remains high, and the threat of antimicrobial-resistant bacteria grows with time, putting us at inferiority. Although tremendous resources have been devoted to the development of antimicrobial agents, we have yet to recover from the lost ground we have been driven into. Looking back at the evolution of treatment for cancer, which, like infectious diseases, has the similarity that host immunity eliminates the lesion, the development of drugs to eliminate the tumor itself has shifted from a single-minded focus on drug development to the establishment of a treatment strategy in which the de-suppression of host immunity is another pillar of treatment. In infectious diseases, on the other hand, the development of therapies that strengthen and support the immune system has only just begun. Among innate immunity, the first line of defense that bacteria encounter after invading the host, the molecular mechanisms of the phagolysosome pathway, which begins with phagocytosis to fusion with lysosome, have been elucidated in detail. Bacteria have a large number of strategies to escape and survive the pathway. Although the full picture is still unfathomable, the molecular mechanisms have been elucidated for some of them, providing sufficient clues for intervention. In this article, we review the host defense mechanisms and bacterial evasion mechanisms and discuss the possibility of host-directed therapy for bacterial infection by intervening in the phagolysosome pathway.

LHPP-mediated inorganic pyrophosphate hydrolysis-driven lysosomal acidification in astrocytes regulates adult neurogenesis

In bacteria, archaea, protists, and plants, the hydrolysis of pyrophosphate (PPi) by inorganic pyrophosphatase (PPase) can, under stress conditions, substitute for ATP-driven proton flux to generate a proton gradient and induce luminal acidification. However, this strategy is considered to be lost in eukaryotes. Here, we report that LHPP, a poorly understood PPase that exhibits activity at acidic pH, is primarily expressed in astrocytes and partly localized on lysosomal membranes. Under stress conditions, LHPP is recruited to vacuolar ATPase (V-ATPase) and facilitates V-ATPase-dependent proton transport and lysosomal acidification by hydrolyzing PPi. LHPP knockout (KO) mice have no discernable phenotype but are resilient to chronic-stress-induced depression-like behaviors. Mechanistically, LHPP deficiency prevents lysosome-dependent degradation of C/EBPβ and induces the expression of a group of chemokines that promote adult neurogenesis. Together, these findings suggest that LHPP is likely to be a therapeutic target for stress-related brain disease.

Lysosomal dysfunction in Down syndrome and Alzheimer mouse models is caused by v-ATPase inhibition by Tyr682-phosphorylated APP βCTF

Lysosome dysfunction arises early and propels Alzheimer's disease (AD). Herein, we show that amyloid precursor protein (APP), linked to early-onset AD in Down syndrome (DS), acts directly via its β-C-terminal fragment (βCTF) to disrupt lysosomal vacuolar (H+)-adenosine triphosphatase (v-ATPase) and acidification. In human DS fibroblasts, the phosphorylated 682YENPTY internalization motif of APP-βCTF binds selectively within a pocket of the v-ATPase V0a1 subunit cytoplasmic domain and competitively inhibits association of the V1 subcomplex of v-ATPase, thereby reducing its activity. Lowering APP-βCTF Tyr682 phosphorylation restores v-ATPase and lysosome function in DS fibroblasts and in vivo in brains of DS model mice. Notably, lowering APP-βCTF Tyr682 phosphorylation below normal constitutive levels boosts v-ATPase assembly and activity, suggesting that v-ATPase may also be modulated tonically by phospho-APP-βCTF. Elevated APP-βCTF Tyr682 phosphorylation in two mouse AD models similarly disrupts v-ATPase function. These findings offer previously unknown insight into the pathogenic mechanism underlying faulty lysosomes in all forms of AD.

Procyanidin B2 3,3''-di-O-gallate ameliorates imiquimod-induced skin inflammation by suppressing TLR7 signaling through the inhibition of endosomal acidification in dendritic cells

The excessive activation of abnormal T helper 17 (Th17) cells and dendritic cells (DCs) in the dermis and epidermis causes severe inflammation of the skin. Toll-like receptor 7 (TLR7)-located in the endosomes of DCs-recognizes nucleic acids from pathogens as well as imiquimod (IMQ), which plays a crucial role in the pathogenesis of skin inflammation. Procyanidin B2 3,3''-di-O-gallate (PCB2DG), a polyphenol, has been reported to suppress the excessive production of proinflammatory cytokines from T cells. The aim of this study was to demonstrate the inhibitory effect of PCB2DG on skin inflammation and TLR7 signaling in DCs. In vivo studies showed that the clinical symptoms of dermatitis were markedly improved by the oral administration of PCB2DG in mouse dermatitis model caused by IMQ application, accompanied by the suppression of excessive cytokine secretion in the inflamed skin and spleen. In vitro, PCB2DG significantly decreased cytokine production in TLR7- or TLR9 ligand-stimulated bone marrow-derived dendritic cells (BMDCs), suggesting that PCB2DG suppresses endosomal toll-like receptors (TLR) signaling in DCs. The activity of endosomal TLRs depends on endosomal acidification, which was significantly inhibited by PCB2DG in BMDCs. The addition of cAMP, an accelerator of endosomal acidification, abrogated the inhibitory effect of cytokine production by PCB2DG. These results provide a new insight into developing functional foods, including PCB2DG, to improve the symptoms of skin inflammation through the suppression of TLR7 signaling in DCs.

Neutrophil CEACAM1 determines susceptibility to NETosis by regulating the S1PR2/S1PR3 axis in liver transplantation

Neutrophils, the largest innate immune cell population in humans, are the primary proinflammatory sentinel in the ischemia-reperfusion injury (IRI) mechanism in orthotopic liver transplantation (OLT). Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1, CC1, or CD66a) is essential in neutrophil activation and serves as a checkpoint regulator of innate immune-driven IRI cascade in OLT. Although CC1 alternative splicing generates two functionally distinct short and long cytoplasmic isoforms, their role in neutrophil activation remains unknown. Here, we undertook molecular and functional studies to interrogate the significance of neutrophil CC1 signaling in mouse and human OLT recipients. In the experimental arm, we employed a mouse OLT model to document that ablation of recipient-derived neutrophil CC1-long (CC1-L) isotype aggravated hepatic IRI by promoting neutrophil extracellular traps (NETs). Notably, by regulating the S1P-S1PR2/S1PR3 axis, neutrophil CC1-L determined susceptibility to NET formation via autophagy signaling. In the clinical arm, liver grafts from 55 transplant patients selectively enriched for neutrophil CC1-L showed relative resistance to ischemia-reperfusion (IR) stress/tissue damage, improved hepatocellular function, and clinical outcomes. In conclusion, despite neutrophils being considered a principal villain in peritransplant tissue injury, their CC1-L isoform may serve as a regulator of IR stress resistance/NETosis in human and mouse OLT recipients.

Avoid being trapped by your liver: ischemia-reperfusion injury in liver transplant triggers S1P-mediated NETosis

Liver transplantation can be a life-saving treatment for end-stage hepatic disease. Unfortunately, some recipients develop ischemia-reperfusion injury (IRI) that leads to poor short- and long-term outcomes. Recent work has shown neutrophils contribute to IRI by undergoing NETosis, a form of death characterized by DNA ejection resulting in inflammatory extracellular traps. In this issue of the JCI, Hirao and Kojima et al. report that sphingosine-1-phosphate (S1P) expression induced by liver transplant-mediated IRI triggers NETosis. They also provide evidence that neutrophil expression of the carcinoembryonic antigen-related cell adhesion molecule-1 (CC1) long isoform inhibited NETosis by controlling S1P receptor-mediated autophagic flux. These findings suggest stimulating regulatory mechanisms that suppress NETosis could be used to prevent IRI.

Ras-mutant cancers are sensitive to small molecule inhibition of V-type ATPases in mice

Mutations in Ras family proteins are implicated in 33% of human cancers, but direct pharmacological inhibition of Ras mutants remains challenging. As an alternative to direct inhibition, we screened for sensitivities in Ras-mutant cells and discovered 249C as a Ras-mutant selective cytotoxic agent with nanomolar potency against a spectrum of Ras-mutant cancers. 249C binds to vacuolar (V)-ATPase with nanomolar affinity and inhibits its activity, preventing lysosomal acidification and inhibiting autophagy and macropinocytosis pathways that several Ras-driven cancers rely on for survival. Unexpectedly, potency of 249C varies with the identity of the Ras driver mutation, with the highest potency for KRASG13D and G12V both in vitro and in vivo, highlighting a mutant-specific dependence on macropinocytosis and lysosomal pH. Indeed, 249C potently inhibits tumor growth without adverse side effects in mouse xenografts of KRAS-driven lung and colon cancers. A comparison of isogenic SW48 xenografts with different KRAS mutations confirmed that KRASG13D/+ (followed by G12V/+) mutations are especially sensitive to 249C treatment. These data establish proof-of-concept for targeting V-ATPase in cancers driven by specific KRAS mutations such as KRASG13D and G12V.

Direct control of lysosomal catabolic activity by mTORC1 through regulation of V-ATPase assembly

Mammalian cells can acquire exogenous amino acids through endocytosis and lysosomal catabolism of extracellular proteins. In amino acid-replete environments, nutritional utilization of extracellular proteins is suppressed by the amino acid sensor mechanistic target of rapamycin complex 1 (mTORC1) through an unknown process. Here, we show that mTORC1 blocks lysosomal degradation of extracellular proteins by suppressing V-ATPase-mediated acidification of lysosomes. When mTORC1 is active, peripheral V-ATPase V₁ domains reside in the cytosol where they are stabilized by association with the chaperonin TRiC. Consequently, most lysosomes display low catabolic activity. When mTORC1 activity declines, V-ATPase V₁ domains move to membrane-integral V-ATPase V_o domains at lysosomes to assemble active proton pumps. The resulting drop in luminal pH increases protease activity and degradation of protein contents throughout the lysosomal population. These results uncover a principle by which cells rapidly respond to changes in their nutrient environment by mobilizing the latent catabolic capacity of lysosomes.

mTORC1 blocks lysosomal nutrient generation. Here, the authors show that mTORC1 inactivation triggers V-ATPase assembly, which rapidly initiates lysosomal acidification and degradation of protein contents throughout the lysosomal population.

Structure of V-ATPase from citrus fruit

We used the Legionella pneumophila effector SidK to affinity purify the endogenous vacuolar-type ATPases (V-ATPases) from lemon fruit. The preparation was sufficient for cryoelectron microscopy, allowing structure determination of the enzyme in two rotational states. The structure defines the ATP:H⁺ ratio of the enzyme, demonstrating that it can establish a maximum ΔpH of ∼3, which is insufficient to maintain the low pH observed in the vacuoles of juice sac cells in lemons and other citrus fruit. Compared with yeast and mammalian enzymes, the membrane region of the plant V-ATPase lacks subunit f and possesses an unusual configuration of transmembrane α helices. Subunit H, which inhibits ATP hydrolysis in the isolated catalytic region of V-ATPase, adopts two different conformations in the intact complex, hinting at a role in modulating activity in the intact enzyme.

V-ATPase is a universal regulator of LC3-associated phagocytosis and non-canonical autophagy

Non-canonical autophagy is a key cellular pathway in immunity, cancer, and neurodegeneration, characterized by conjugation of ATG8 to endolysosomal single membranes (CASM). CASM is activated by engulfment (endocytosis, phagocytosis), agonists (STING, TRPML1), and infection (influenza), dependent on K490 in the ATG16L1 WD40-domain. However, factors associated with non-canonical ATG16L1 recruitment and CASM induction remain unknown. Here, using pharmacological inhibitors, we investigate a role for V-ATPase during non-canonical autophagy. We report that increased V0–V1 engagement is associated with, and sufficient for, CASM activation. Upon V0–V1 binding, V-ATPase recruits ATG16L1, via K490, during LC3-associated phagocytosis (LAP), STING- and drug-induced CASM, indicating a common mechanism. Furthermore, during LAP, key molecular players, including NADPH oxidase/ROS, converge on V-ATPase. Finally, we show that LAP is sensitive to Salmonella SopF, which disrupts the V-ATPase–ATG16L1 axis and provide evidence that CASM contributes to the Salmonella host response. Together, these data identify V-ATPase as a universal regulator of CASM and indicate that SopF evolved in part to evade non-canonical autophagy.

Low-dose metformin targets the lysosomal AMPK pathway through PEN2

Metformin, the most prescribed antidiabetic medicine, has shown other benefits such as anti-ageing and anticancer effects^1-4. For clinical doses of metformin, AMP-activated protein kinase (AMPK) has a major role in its mechanism of action^4,5; however, the direct molecular target of metformin remains unknown. Here we show that clinically relevant concentrations of metformin inhibit the lysosomal proton pump v-ATPase, which is a central node for AMPK activation following glucose starvation⁶. We synthesize a photoactive metformin probe and identify PEN2, a subunit of γ-secretase⁷, as a binding partner of metformin with a dissociation constant at micromolar levels. Metformin-bound PEN2 forms a complex with ATP6AP1, a subunit of the v-ATPase⁸, which leads to the inhibition of v-ATPase and the activation of AMPK without effects on cellular AMP levels. Knockout of PEN2 or re-introduction of a PEN2 mutant that does not bind ATP6AP1 blunts AMPK activation. In vivo, liver-specific knockout of Pen2 abolishes metformin-mediated reduction of hepatic fat content, whereas intestine-specific knockout of Pen2 impairs its glucose-lowering effects. Furthermore, knockdown of pen-2 in Caenorhabditis elegans abrogates metformin-induced extension of lifespan. Together, these findings reveal that metformin binds PEN2 and initiates a signalling route that intersects, through ATP6AP1, the lysosomal glucose-sensing pathway for AMPK activation. This ensures that metformin exerts its therapeutic benefits in patients without substantial adverse effects.

The V-ATPases in cancer and cell death

Transmembrane ATPases are membrane-bound enzyme complexes and ion transporters that can be divided into F-, V-, and A-ATPases according to their structure. The V-ATPases, also known as H⁺-ATPases, are large multi-subunit protein complexes composed of a peripheral domain (V1) responsible for the hydrolysis of ATP and a membrane-integrated domain (V0) that transports protons across plasma membrane or organelle membrane. V-ATPases play a fundamental role in maintaining pH homeostasis through lysosomal acidification and are involved in modulating various physiological and pathological processes, such as macropinocytosis, autophagy, cell invasion, and cell death (e.g., apoptosis, anoikis, alkaliptosis, ferroptosis, and lysosome-dependent cell death). In addition to participating in embryonic development, V-ATPase pathways, when dysfunctional, are implicated in human diseases, such as neurodegenerative diseases, osteopetrosis, distal renal tubular acidosis, and cancer. In this review, we summarize the structure and regulation of isoforms of V-ATPase subunits and discuss their context-dependent roles in cancer biology and cell death. Updated knowledge about V-ATPases may enable us to design new anticancer drugs or strategies.

Purification of active human vacuolar H+-ATPase in native lipid-containing nanodiscs

Vacuolar H+-ATPases (V-ATPases) are large, multisubunit proton pumps that acidify the lumen of organelles in virtually every eukaryotic cell and in specialized acid-secreting animal cells, the enzyme pumps protons into the extracellular space. In higher organisms, most of the subunits are expressed as multiple isoforms, with some enriched in specific compartments or tissues and others expressed ubiquitously. In mammals, subunit a is expressed as four isoforms (a1-4) that target the enzyme to distinct biological membranes. Mutations in a isoforms are known to give rise to tissue-specific disease, and some a isoforms are upregulated and mislocalized to the plasma membrane in invasive cancers. However, isoform complexity and low abundance greatly complicate purification of active human V-ATPase, a prerequisite for developing isoform-specific therapeutics. Here, we report the purification of an active human V-ATPase in native lipid nanodiscs from a cell line stably expressing affinity-tagged a isoform 4 (a4). We find that exogenous expression of this single subunit in HEK293F cells permits assembly of a functional V-ATPase by incorporation of endogenous subunits. The ATPase activity of the preparation is >95% sensitive to concanamycin A, indicating that the lipid nanodisc-reconstituted enzyme is functionally coupled. Moreover, this strategy permits purification of the enzyme's isolated membrane subcomplex together with biosynthetic assembly factors coiled-coil domain-containing protein 115, transmembrane protein 199, and vacuolar H+-ATPase assembly integral membrane protein 21. Our work thus lays the groundwork for biochemical characterization of active human V-ATPase in an a subunit isoform-specific manner and establishes a platform for the study of the assembly and regulation of the human holoenzyme.

Live imaging of intra-lysosome pH in cell lines and primary neuronal culture using a novel genetically encoded biosensor

Disorders of lysosomal physiology have increasingly been found to underlie the pathology of a rapidly growing cast of neurodevelopmental disorders and sporadic diseases of aging. One cardinal aspect of lysosomal (dys)function is lysosomal acidification in which defects trigger lysosomal stress signaling and defects in proteolytic capacity. We have developed a genetically encoded ratiometric probe to measure lysosomal pH coupled with a purification tag to efficiently purify lysosomes for both proteomic and in vitro evaluation of their function. Using our probe, we showed that lysosomal pH is remarkably stable over a period of days in a variety of cell types. Additionally, this probe can be used to determine that lysosomal stress signaling via TFEB is uncoupled from gross changes in lysosomal pH. Finally, we demonstrated that while overexpression of ARL8B GTPase causes striking alkalinization of peripheral lysosomes in HEK293 T cells, peripheral lysosomes per se are no less acidic than juxtanuclear lysosomes in our cell lines.Abbreviations: ARL8B: ADP ribosylation factor like GTPase 8B; ATP: adenosine triphosphate; ATP5F1B/ATPB: ATP synthase F1 subunit beta; ATP6V1A: ATPase H+ transporting V1 subunit A; Baf: bafilomycin A1; BLOC-1: biogenesis of lysosome-related organelles complex 1; BSA: bovine serum albumin; Cos7: African green monkey kidney fibroblast-like cell line; CQ: chloroquine; CTSB: cathepsin B; CYCS: cytochrome c, somatic; DAPI: 4',6-diamidino -2- phenylindole; DIC: differential interference contrast; DIV: days in vitro; DMEM: Dulbecco's modified Eagle's medium;‎ E8: embryonic day 8; EEA1: early endosome antigen 1; EGTA: ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid; ER: endoplasmic reticulum; FBS: fetal bovine serum; FITC: fluorescein isothiocyanate; GABARAPL2: GABA type A receptor associated protein like 2; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GOLGA2/GM130: golgin A2; GTP: guanosine triphosphate; HEK293T: human embryonic kidney 293 cells, that expresses a mutant version of the SV40 large T antigen; HeLa: Henrietta Lacks-derived cell; HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; HRP: horseradish peroxidase; IGF2R/ciM6PR: insulin like growth factor 2 receptor; LAMP1/2: lysosomal associated membrane protein 1/2; LMAN2/VIP36: lectin, mannose binding 2; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MTORC1: mechanistic target of rapamycin kinase complex 1; PCR: polymerase chain reaction; PDL: poly-d-lysine; PGK1p: promotor from human phosphoglycerate kinase 1; PIKFYVE: phosphoinositide kinase, FYVE-type zinc finger containing; PPT1/CLN1: palmitoyl-protein thioesterase 1; RPS6KB1/p70: ribosomal protein S6 kinase B1; STAT3: signal transducer and activator of transcription 3; TAX1BP1: Tax1 binding protein 1; TFEB: transcription factor EB; TGN: trans-Golgi network; TGOLN2/TGN46: trans-Golgi network protein 2; TIRF: total internal reflection fluorescence; TMEM106B: transmembrane protein 106B; TOR: target of rapamycin; TRPM2: transient receptor potential cation channel subfamily M member 2; V-ATPase: vacuolar-type proton-translocating ATPase; VPS35: VPS35 retromer complex component.

Defining steps in RAVE-catalyzed V-ATPase assembly using purified RAVE and V-ATPase subcomplexes

The vacuolar H⁺-ATPase (V-ATPase) is a highly conserved proton pump responsible for the acidification of intracellular organelles in virtually all eukaryotic cells. V-ATPases are regulated by the rapid and reversible disassembly of the peripheral V₁ domain from the integral membrane V_o domain, accompanied by release of the V₁ C subunit from both domains. Efficient reassembly of V-ATPases requires the Regulator of the H⁺-ATPase of Vacuoles and Endosomes (RAVE) complex in yeast. Although a number of pairwise interactions between RAVE and V-ATPase subunits have been mapped, the low endogenous levels of the RAVE complex and lethality of constitutive RAV1 overexpression have hindered biochemical characterization of the intact RAVE complex. We describe a novel inducible overexpression system that allows purification of native RAVE and RAVE–V₁ complexes. Both purified RAVE and RAVE–V₁ contain substoichiometric levels of subunit C. RAVE–V₁ binds tightly to expressed subunit C in vitro, but binding of subunit C to RAVE alone is weak. Neither RAVE nor RAVE–V₁ interacts with the N-terminal domain of V_o subunit Vph1 in vitro. RAVE–V₁ complexes, like isolated V₁, have no MgATPase activity, suggesting that RAVE cannot reverse V₁ inhibition generated by rotation of subunit H and entrapment of MgADP that occur upon disassembly. However, purified RAVE can accelerate reassembly of V₁ carrying a mutant subunit H incapable of inhibition with V_o complexes reconstituted into lipid nanodiscs, consistent with its catalytic activity in vivo. These results provide new insights into the possible order of events in V-ATPase reassembly and the roles of the RAVE complex in each event.

The Regulatory Function of CCR9+ Dendritic Cells in Inflammation and Autoimmunity

Chemokine receptor CCR9 is a G protein–coupled receptor and expressed on several types of immune cells, including dendritic cells (DCs), CD4⁺ T cells, and B cells. CCR9 drives the migration of immune cells to gradients of its cognate ligand CCL25. The chemokine CCL25 is mostly produced by gut and thymic epithelial cells. Gut- and thymic-homing DCs are known to express CCR9, and these cells are predominantly localized in the gut lining and thymus. CCR9⁺ DCs are implicated in regulating inflammation, food allergy, alloimmunity, and autoimmunity. Differential interaction of CCR9⁺ DCs with lymphoid and myeloid cells in the thymus, secondary lymphoid tissues, and mucosal sites offer crucial insights to immune regulation. In this review, we examine the phenotypes, distributions, and interactions of CCR9⁺ DCs with other immune cells, elucidating their functions and role in inflammation and autoimmunity.

AKT Ser/Thr kinase increases V-ATPase-dependent lysosomal acidification in response to amino acid starvation in mammalian cells

The vacuolar H+-ATPase (V-ATPase) is an ATP-dependent proton pump that is essential for cellular homeostasis. V-ATPase activity is controlled by the regulated assembly of the enzyme from its component V1 and V0 domains. We previously reported that amino acid starvation rapidly increases V-ATPase assembly and activity in mammalian lysosomes, but the signaling pathways controlling this effect are unknown. In testing inhibitors of pathways important for controlling cellular metabolism, we found here that the cAMP-dependent protein kinase (PKA) inhibitor H89 increases lysosomal V-ATPase activity and blocks any further change upon starvation. The AMP-activated protein kinase (AMPK) inhibitor dorsomorphin decreased lysosomal V-ATPase activity and also blocked any increase upon starvation. However, CRISPR-mediated gene editing revealed that PKA and AMPK are not required for the starvation-dependent increase in lysosomal V-ATPase activity, indicating that H89 and dorsomorphin modify V-ATPase activity through other cellular targets. We next found that the AKT Ser/Thr kinase (AKT) inhibitor MK2206 blocks the starvation-dependent increase in lysosomal V-ATPase activity without altering basal activity. Expression of AKT1 or AKT3, but not AKT2, was required for increased lysosomal V-ATPase activity in response to amino acid starvation in mouse fibroblasts. Finally, HEK293T cells expressing only AKT1 responded normally to starvation, whereas cells expressing only AKT2 displayed a significantly reduced increase in V-ATPase activity and assembly upon starvation. These results show that AKT is required for controlling the rapid response of lysosomal V-ATPase activity to changes in amino acid availability and that this response depends on specific AKT isoforms.

Pharmacological Targeting of Vacuolar H+-ATPase via Subunit V1G Combats Multidrug-Resistant Cancer

Multidrug resistance (MDR) in cancer remains a major challenge for the success of chemotherapy. Natural products have been a rich source for the discovery of drugs against MDR cancers. Here, we applied high-throughput cytotoxicity screening of an in-house natural product library against MDR SGC7901/VCR cells and identified that the cyclodepsipeptide verucopeptin demonstrated notable antitumor potency. Cytological profiling combined with click chemistry-based proteomics revealed that ATP6V1G directly interacted with verucopeptin. ATP6V1G, a subunit of the vacuolar H⁺-ATPase (v-ATPase) that has not been previously targeted, was essential for SGC7901/VCR cell growth. Verucopeptin exhibited strong inhibition of both v-ATPase activity and mTORC1 signaling, leading to substantial pharmacological efficacy against SGC7901/VCR cell proliferation and tumor growth in vivo. Our results demonstrate that targeting v-ATPase via its V1G subunit constitutes a unique approach for modulating v-ATPase and mTORC1 signaling with great potential for the development of therapeutics against MDR cancers.

Regulation of V-ATPase Activity and Organelle pH by Phosphatidylinositol Phosphate Lipids

Luminal pH and the distinctive distribution of phosphatidylinositol phosphate (PIP) lipids are central identifying features of organelles in all eukaryotic cells that are also critical for organelle function. V-ATPases are conserved proton pumps that populate and acidify multiple organelles of the secretory and the endocytic pathway. Complete loss of V-ATPase activity causes embryonic lethality in higher animals and conditional lethality in yeast, while partial loss of V-ATPase function is associated with multiple disease states. On the other hand, many cancer cells increase their virulence by upregulating V-ATPase expression and activity. The pH of individual organelles is tightly controlled and essential for function, but the mechanisms for compartment-specific pH regulation are not completely understood. There is substantial evidence indicating that the PIP content of membranes influences organelle pH. We present recent evidence that PIPs interact directly with subunit isoforms of the V-ATPase to dictate localization of V-ATPase subpopulations and participate in their regulation. In yeast cells, which have only one set of organelle-specific V-ATPase subunit isoforms, the Golgi-enriched lipid PI(4)P binds to the cytosolic domain of the Golgi-enriched a-subunit isoform Stv1, and loss of PI(4)P binding results in mislocalization of Stv1-containing V-ATPases from the Golgi to the vacuole/lysosome. In contrast, levels of the vacuole/lysosome-enriched signaling lipid PI(3,5)P₂ affect assembly and activity of V-ATPases containing the Vph1 a-subunit isoform. Mutations in the Vph1 isoform that disrupt the lipid interaction increase sensitivity to stress. These studies have decoded “zip codes” for PIP lipids in the cytosolic N-terminal domain of the a-subunit isoforms of the yeast V-ATPase, and similar interactions between PIP lipids and the V-ATPase subunit isoforms are emerging in higher eukaryotes. In addition to direct effects on the V-ATPase, PIP lipids are also likely to affect organelle pH indirectly, through interactions with other membrane transporters. We discuss direct and indirect effects of PIP lipids on organelle pH, and the functional consequences of the interplay between PIP lipid content and organelle pH.

Regulation and function of V-ATPases in physiology and disease

The vacuolar H⁺-ATPases (V-ATPases) are essential, ATP-dependent proton pumps present in a variety of eukaryotic cellular membranes. Intracellularly, V-ATPase-dependent acidification functions in such processes as membrane traffic, protein degradation, autophagy and the coupled transport of small molecules. V-ATPases at the plasma membrane of certain specialized cells function in such processes as bone resorption, sperm maturation and urinary acidification. V-ATPases also function in disease processes such as pathogen entry and cancer cell invasiveness, while defects in V-ATPase genes are associated with disorders such as osteopetrosis, renal tubular acidosis and neurodegenerative diseases. This review highlights recent advances in our understanding of V-ATPase structure, mechanism, function and regulation, with an emphasis on the signaling pathways controlling V-ATPase assembly in mammalian cells. The role of V-ATPases in cancer and other human pathologies, and the prospects for therapeutic intervention, are also discussed.

Endoplasmic Reticulum-Associated Degradation-Dependent Processing in Cross-Presentation and Its Potential for Dendritic Cell Vaccinations: A Review

While the success of dendritic cell (DC) vaccination largely depends on cross-presentation (CP) efficiency, the precise molecular mechanism of CP is not yet characterized. Recent research revealed that endoplasmic reticulum (ER)-associated degradation (ERAD), which was first identified as part of the protein quality control system in the ER, plays a pivotal role in the processing of extracellular proteins in CP. The discovery of ERAD-dependent processing strongly suggests that the properties of extracellular antigens are one of the keys to effective DC vaccination, in addition to DC subsets and the maturation of these cells. In this review, we address recent advances in CP, focusing on the molecular mechanisms of the ERAD-dependent processing of extracellular proteins. As ERAD itself and the ERAD-dependent processing in CP share cellular machinery, enhancing the recognition of extracellular proteins, such as the ERAD substrate, by ex vivo methods may serve to improve the efficacy of DC vaccination.

Enhanced translation expands the endo-lysosome size and promotes antigen presentation during phagocyte activation

The mechanisms that govern organelle adaptation and remodelling remain poorly defined. The endo-lysosomal system degrades cargo from various routes, including endocytosis, phagocytosis, and autophagy. For phagocytes, endosomes and lysosomes (endo-lysosomes) are kingpin organelles because they are essential to kill pathogens and process and present antigens. During phagocyte activation, endo-lysosomes undergo a morphological transformation, going from a collection of dozens of globular structures to a tubular network in a process that requires the phosphatidylinositol-3-kinase-AKT-mechanistic target of rapamycin (mTOR) signalling pathway. Here, we show that the endo-lysosomal system undergoes an expansion in volume and holding capacity during phagocyte activation within 2 h of lipopolysaccharides (LPS) stimulation. Endo-lysosomal expansion was paralleled by an increase in lysosomal protein levels, but this was unexpectedly largely independent of the transcription factor EB (TFEB) and transcription factor E3 (TFE3), which are known to scale up lysosome biogenesis. Instead, we demonstrate a hitherto unappreciated mechanism of acute organelle expansion via mTOR Complex 1 (mTORC1)-dependent increase in translation, which appears to be mediated by both S6Ks and 4E-BPs. Moreover, we show that stimulation of RAW 264.7 macrophage cell line with LPS alters translation of a subset but not all of mRNAs encoding endo-lysosomal proteins, thereby suggesting that endo-lysosome expansion is accompanied by functional remodelling. Importantly, mTORC1-dependent increase in translation activity was necessary for efficient and rapid antigen presentation by dendritic cells. Collectively, we identified a previously unknown and functionally relevant mechanism for endo-lysosome expansion that relies on mTORC1-dependent translation to stimulate endo-lysosome biogenesis in response to an infection signal.

Activation of phagocytes rapidly expands the endo-lysosomal system and promotes antigen presentation. Endo-lysosome expansion was driven by mTORC1-dependent enhanced translation, revealing regulated translation as a mechanism to remodel membrane organelles in response to external signals and stresses.

Heat shock factor 4 regulates lysosome activity by modulating the αB-crystallin-ATP6V1A-mTOR complex in ocular lens

BACKGROUND

Germline mutations in heat shock factor 4 (HSF4) cause congenital cataracts. Previously, we have shown that HSF4 is involved in regulating lysosomal pH in mouse lens epithelial cell in vitro. However, the underlying mechanism remains unclear.

METHODS

HSF4-deficient mouse lens epithelial cell lines and zebrafish were used in this study. Immunoblotting and quantitative RT-PCR were used for expression analysis. The protein-protein interactions were tested with GST-pull downs. The lysosomes were fractioned by ultracentrifugation.

RESULTS

HSF4 deficiency or knock down of αB-crystallin elevates lysosomal pH and increases the ubiquitination and degradation of ATP6V1A by the proteasome. αB-crystallin localizes partially in the lysosome and interacts solely with the ATP6V1A protein of the V1 complex of V-ATPase. Furthermore, αB-crystallin can co-precipitate with mTORC1 and ATP6V1A in GST pull down assays. Inhibition of mTORC1 by rapamycin or siRNA can lead to dissociation of αB-crystallin from the ATP6V1A and mTORC1complex, shortening the half-life of ATP6V1A and increasing the lysosomal pH. Mutation of ATP6V1A/S441A (the predicted mTOR phosphorylation site) reduces its association with αB-crystallin. In the zebrafish model, HSF4 deficiency reduces αB-crystallin expression and elevates the lysosomal pH in lens tissues.

CONCLUSION

HSF4 regulates lysosomal acidification by controlling the association of αB-crystallin with ATP6V1A and mTOR and regulating ATP6V1A protein stabilization.

GENERAL SIGNIFICANCE

This study uncovers a novel function of αB-crystallin, demonstrating that αB-crystallin can regulate lysosomal ATP6V1A protein stabilization by complexing to ATP6V1A and mTOR. This highlights a novel mechanism by which HSF4 regulates the proteolytic process of organelles during lens development.

Oxygen Tension Regulates Lysosomal Activation and Receptor Tyrosine Kinase Degradation

Oxygen sensing is crucial for adaptation to variable habitats and physiological conditions. Low oxygen tension, or hypoxia, is a common feature of solid tumors, and hypoxic tumors are often more aggressive and resistant to therapy. Here we show that, in cultured mammalian cells, hypoxia suppressed lysosomal acidification/activation and receptor tyrosine kinase (RTK) degradation. Hypoxia down-regulated mTORc1, reducing its ability to activate transcription factor EB (TFEB), a master regulator of V-ATPase, the lysosomal proton pump. Hypoxia prevented epidermal growth factor receptor (EGFR) degradation in tumor tissues, whereas activation of lysosomes enhanced tumor cell response to anti-EGFR treatment. Our results link oxygen tension and lysosomal activity, provide a molecular explanation of the malignant phenotype associated with hypoxic tumors, and suggest activation of lysosomes may provide therapeutic benefit in RTK-targeted cancer therapy.

Transient Receptor Potential V Channels Are Essential for Glucose Sensing by Aldolase and AMPK

Fructose-1,6-bisphosphate (FBP) aldolase links sensing of declining glucose availability to AMPK activation via the lysosomal pathway. However, how aldolase transmits lack of occupancy by FBP to AMPK activation remains unclear. Here, we show that FBP-unoccupied aldolase interacts with and inhibits endoplasmic reticulum (ER)-localized transient receptor potential channel subfamily V, inhibiting calcium release in low glucose. The decrease of calcium at contact sites between ER and lysosome renders the inhibited TRPV accessible to bind the lysosomal v-ATPase that then recruits AXIN:LKB1 to activate AMPK independently of AMP. Genetic depletion of TRPVs blocks glucose starvation-induced AMPK activation in cells and liver of mice, and in nematodes, indicative of physical requirement of TRPVs. Pharmacological inhibition of TRPVs activates AMPK and elevates NAD+ levels in aged muscles, rejuvenating the animals' running capacity. Our study elucidates that TRPVs relay the FBP-free status of aldolase to the reconfiguration of v-ATPase, leading to AMPK activation in low glucose.

The Small GTPase Arf6: An Overview of Its Mechanisms of Action and of Its Role in Host⁻Pathogen Interactions and Innate Immunity

The small GTase Arf6 has several important functions in intracellular vesicular trafficking and regulates the recycling of different types of cargo internalized via clathrin-dependent or -independent endocytosis. It activates the lipid modifying enzymes PIP 5-kinase and phospholipase D, promotes actin polymerization, and affects several functionally distinct processes in the cell. Arf6 is used for the phagocytosis of pathogens and can be directly or indirectly targeted by various pathogens to block phagocytosis or induce the uptake of intracellular pathogens. Arf6 is also used in the signaling of Toll-like receptors and in the activation of NADPH oxidases. In this review, we first give an overview of the different roles and mechanisms of action of Arf6 and then focus on its role in innate immunity and host–pathogen interactions.

Dendritic cells are what they eat: how their metabolism shapes T helper cell polarization

Dendritic cells (DCs) are professional antigen-presenting cells that play a crucial role in the priming and differentiation of CD4⁺ T cells into several distinct subsets including effector T helper (Th) 1, Th17 and Th2 cells, as well as regulatory T cells (Tregs). It is becoming increasingly clear that cellular metabolism shapes the functional properties of DCs. Specifically, the ability of DCs to drive polarization of different Th cell subsets may be orchestrated by the engagement of distinct metabolic pathways. In this review, we will discuss the recent advances in the DC metabolism field, by focusing on how cellular metabolism of DCs shapes their priming and polarization of distinct Th cell responses.

Regulation of Dendritic Cell Immune Function and Metabolism by Cellular Nutrient Sensor Mammalian Target of Rapamycin (mTOR)

Dendritic cell (DC) activation is characterized by an acute increase in glucose metabolic flux that is required to fuel the high anabolic rates associated with DC activation. Inhibition of glycolysis significantly attenuates most aspects of DC immune effector function including antigen presentation, inflammatory cytokine production, and T cell stimulatory capacity. The cellular nutrient sensor mammalian/mechanistic Target of Rapamycin (mTOR) is an important upstream regulator of glycolytic metabolism and plays a central role in coordinating DC metabolic changes and immune responses. Because mTOR signaling can be activated by a variety of immunological stimuli, including signaling through the Toll-like Receptor (TLR) family of receptors, mTOR is involved in orchestrating many aspects of the DC metabolic response to microbial stimuli. It has become increasingly clear that mTOR's role in promoting or attenuating inflammatory processes in DCs is highly context-dependent and varies according to specific cellular subsets and the immunological conditions being studied. This review will address key aspects of the complex role of mTOR in regulating DC metabolism and effector function.

Large-Scale Proteomic Identification of Targets of Cellular miR-197 Downregulated by Enterovirus A71

MicroRNAs are noncoding RNA species comprising 18-23 nucleotides that regulate host-virus interaction networks. Here, we show that enterovirus A71 infection in human rhabdomyosarcoma (RD) is regulated by miR-197 expression. Transfection of miR-197 mimic into RD cells inhibited virus replication by interfering with the viral RNA synthesis. We employed a combination of mass-spectrometry-based quantitative proteomics with the stable isotope labeling with amino acids in cell culture (SILAC) approach for the identification of the miR-197 target genes in RD cells and to investigate the differential expression of the prospective target proteins. A total of 1822 proteins were repeatedly identified in miR-197-transfected RD cells, 106 of which were predicted to have seed sites by TargetScan. Notably, seven of eight selected genes potentially related to viral replication and immune response were validated as direct miR-197 targets, using a luciferase 3'-untranslated region (UTR) reporter assay. The expression levels of three selected endogenous molecules (ITGAV, ETF1, and MAP2K1/MEK1) were significantly reduced when RD cells were transfected with a miR-197 mimic. Our results provide a comprehensive database of miR-197 targets, which might provide better insights into the understanding of host-virus interaction.

Regulation of V-ATPase Assembly in Nutrient Sensing and Function of V-ATPases in Breast Cancer Metastasis

V-ATPases are proton pumps that function to acidify intracellular compartments in all eukaryotic cells, and to transport protons across the plasma membrane of certain specialized cells. V-ATPases function in many normal and disease processes, including membrane traffic, protein degradation, pathogen entry, and cancer cell invasion. An important mechanism of regulating V-ATPase activity in vivo is regulated assembly, which is the reversible dissociation of the ATP-hydrolytic V₁ domain from the proton-conducting V₀ domain. Regulated assembly is highly conserved and occurs in response to various nutrient cues, suggesting that it plays an important role in cellular homeostasis. We have recently found that starvation of mammalian cells for either amino acids or glucose increases V-ATPase assembly on lysosomes, possibly to increase protein degradation (for amino acid homeostasis) or for the utilization of alternative energy sources (during glucose starvation). While regulation of assembly in response to amino acid starvation does not involve PI3K or mTORC1, glucose-regulated assembly involves both PI3K and AMPK. Another important form of V-ATPase regulation is the targeting of the enzyme to different cellular membranes, which is controlled by isoforms of subunit a. We have shown that V-ATPases are localized to the plasma membrane of highly invasive breast cancer cells, where they promote cell migration and invasion. Furthermore, overexpression of the a3 isoform is responsible for plasma membrane targeting of V-ATPases in breast tumor cells leading to their increased invasiveness.

V-ATPase-dependent repression of androgen receptor in prostate cancer cells

Prostate Cancer (PCa) is the most commonly diagnosed cancer and the third leading cause of death for men in the United States. Suppression of androgen receptor (AR) expression is a desirable mechanism to manage PCa. Our studies showed that AR expression was reduced in LAPC4 and LNCaP PCa cell lines treated with nanomolar concentrations of the V-ATPase inhibitor concanamycin A (CCA). This treatment decreased PSA mRNA levels, indicative of reduced AR activity. V-ATPase-dependent repression of AR expression was linked to defective endo-lysosomal pH regulation and reduced AR expression at the transcriptional level. CCA treatment increased the protein level and nuclear localization of the alpha subunit of the transcription factor HIF-1 (HIF-1α) in PCa cells via decreased hydroxylation and degradation of HIF-1α. The addition of iron (III) citrate restored HIF-1α hydroxylation and decreased total HIF-1α levels in PCa cells treated with CCA. Moreover, iron treatment partially rescued CCA-mediated AR repression. Dimethyloxalylglycine (DMOG), which prevents HIF-1α degradation independently of V-ATPase, also decreased AR levels, supporting our hypothesis that HIF-1α serves as a downstream mediator in the V-ATPase-AR axis. We propose a new V-ATPase-dependent mechanism to inhibit androgen receptor expression in prostate cancer cells involving defective endosomal trafficking of iron and the inhibition of HIF-1 α-subunit turnover.

Stimulation of TLR3 triggers release of lysosomal ATP in astrocytes and epithelial cells that requires TRPML1 channels

Cross-reactions between innate immunity, lysosomal function, and purinergic pathways may link signaling systems in cellular pathologies. We found activation of toll-like receptor 3 (TLR3) triggers lysosomal ATP release from both astrocytes and retinal pigmented epithelial (RPE) cells. ATP efflux was accompanied by lysosomal acid phosphatase and beta hexosaminidase release. Poly(I:C) alkalinized lysosomes, and lysosomal alkalization with bafilomycin or chloroquine triggered ATP release. Lysosomal rupture with glycyl-L-phenylalanine-2-naphthylamide (GPN) eliminated both ATP and acid phosphatase release. Secretory lysosome marker LAMP3 colocalized with VNUT, while MANT-ATP colocalized with LysoTracker. Unmodified membrane-impermeant 21-nt and "non-targeting" scrambled 21-nt siRNA triggered ATP and acid phosphatase release, while smaller 16-nt RNA was ineffective. Poly(I:C)-dependent ATP release was reduced by TBK-1 block and in TRPML1-/- cells, while TRPML activation with ML-SA1 was sufficient to release both ATP and acid phosphatase. The ability of poly(I:C) to raise cytoplasmic Ca2+ was abolished by removing extracellular ATP with apyrase, suggesting ATP release by poly(I:C) increased cellular signaling. Starvation but not rapamycin prevented lysosomal ATP release. In summary, stimulation of TLR3 triggers lysosomal alkalization and release of lysosomal ATP through activation of TRPML1; this links innate immunity to purinergic signaling via lysosomal physiology, and suggests even scrambled siRNA can influence these pathways.

Glucose starvation increases V-ATPase assembly and activity in mammalian cells through AMP kinase and phosphatidylinositide 3-kinase/Akt signaling

The vacuolar H⁺-ATPase (V-ATPase) is an ATP-driven proton pump involved in many cellular processes. An important mechanism by which V-ATPase activity is controlled is the reversible assembly of its two domains, namely the peripheral V₁ domain and the integral V₀ domain. Although reversible assembly is conserved across all eukaryotic organisms, the signaling pathways controlling it have not been fully characterized. Here, we identify glucose starvation as a novel regulator of V-ATPase assembly in mammalian cells. During acute glucose starvation, the V-ATPase undergoes a rapid and reversible increase in assembly and activity as measured by lysosomal acidification. Because the V-ATPase has recently been implicated in the activation of AMP kinase (AMPK), a critical cellular energy sensor that is also activated upon glucose starvation, we compared the time course of AMPK activation and V-ATPase assembly upon glucose starvation. We observe that AMPK activation precedes increased V-ATPase activity. Moreover, the starvation-induced increase in V-ATPase activity and assembly are prevented by the AMPK inhibitor dorsomorphin. These results suggest that increased assembly and activity of the V-ATPase upon glucose starvation are dependent upon AMPK. We also find that the PI3K/Akt pathway, which has previously been implicated in controlling V-ATPase assembly in mammalian cells, also plays a role in the starvation-induced increase in V-ATPase assembly and activity. These studies thus identify a novel stimulus of V-ATPase assembly and a novel signaling pathway involved in regulating this process. The possible function of starvation-induced increase in lysosomal V-ATPase activity is discussed.

Vacuolar ATPase as a possible therapeutic target in human acute myeloid leukemia

INTRODUCTION

V-ATPase is a proton pump expressed both in the membrane of intracellular organelles (e.g. endosomes, lysosomes, Golgi structures) and the plasma membrane. It is an important regulator of organellar functions, intracellular molecular trafficking, intercellular communication and intracellular signaling. It is therefore considered as a possible therapeutic target in the treatment of human malignancies. Areas covered: Relevant publications were identified through literature searches in the PubMed database. We searched for original articles and reviews describing the possible importance of V-ATPase for leukemogenesis and chemosensitivity in human myeloid cells, especially acute myeloid leukemia (AML) cells. Expert commentary: The expression of V-ATPase in the primary human AML cells varies between patients, and high levels are associated with high constitutive release of a wide range of soluble mediators. Several of the molecules included in the V-ATPase interactome may also be important in leukemogenesis and/or development of chemoresistance in human AML. Therapeutic targeting of V-ATPase should therefore be regarded as a possible therapeutic strategy in human AML, but the efficiency of such targeting will probably differ between patients. The possibility of toxicity, especially hematological toxicity and immunosuppression, also has to be clarified.

Activity-independent targeting of mTOR to lysosomes in primary osteoclasts

Mammalian target of rapamycin (mTOR) is activated by numerous stimuli, including amino acids and growth factors. This kinase is part of the mTOR complex 1 (mTORC1) which regulates cell proliferation, differentiation, and autophagy. Active mTORC1 is located on lysosomes and has been reported to disassociate from the lysosomal surface in the absence of amino acids. Furthermore, mTORC1 activity has been linked to the vacuolar H⁺-ATPases (V-ATPases), the proton pumps responsible for lysosomal acidification; however, the exact role of the V-ATPases in mTORC1 signaling is not known. To elucidate the mechanisms involved in mTORC1 regulation by the V-ATPases, we used primary osteoclasts derived from mice carrying a point (R740S) mutation in the a3 subunit of the V-ATPase. In these cells, the mutant protein is expressed but the pump is not functional, resulting in higher lysosomal pH. By analyzing mTOR activation, mTOR/lysosome co-localization, and lysosomal positioning using confocal microscopy, fractionation, and ultrapure lysosomal purification methods, we demonstrate that in primary osteoclasts, mTOR is localized on the lysosomal surface even when mTOR activity is inhibited. Our findings reveal that mTOR targeting to the lysosome in osteoclasts is activity-independent, and that its disassociation from the lysosome during starvation is not universal.

The Function of V-ATPases in Cancer

The vacuolar ATPases (V-ATPases) are a family of proton pumps that couple ATP hydrolysis to proton transport into intracellular compartments and across the plasma membrane. They function in a wide array of normal cellular processes, including membrane traffic, protein processing and degradation, and the coupled transport of small molecules, as well as such physiological processes as urinary acidification and bone resorption. The V-ATPases have also been implicated in a number of disease processes, including viral infection, renal disease, and bone resorption defects. This review is focused on the growing evidence for the important role of V-ATPases in cancer. This includes functions in cellular signaling (particularly Wnt, Notch, and mTOR signaling), cancer cell survival in the highly acidic environment of tumors, aiding the development of drug resistance, as well as crucial roles in tumor cell invasion, migration, and metastasis. Of greatest excitement is evidence that at least some tumors express isoforms of V-ATPase subunits whose disruption is not lethal, leading to the possibility of developing anti-cancer therapeutics that selectively target V-ATPases that function in cancer cells.

Disorders of lysosomal acidification-The emerging role of v-ATPase in aging and neurodegenerative disease

Autophagy and endocytosis deliver unneeded cellular materials to lysosomes for degradation. Beyond processing cellular waste, lysosomes release metabolites and ions that serve signaling and nutrient sensing roles, linking the functions of the lysosome to various pathways for intracellular metabolism and nutrient homeostasis. Each of these lysosomal behaviors is influenced by the intraluminal pH of the lysosome, which is maintained in the low acidic range by a proton pump, the vacuolar ATPase (v-ATPase). New reports implicate altered v-ATPase activity and lysosomal pH dysregulation in cellular aging, longevity, and adult-onset neurodegenerative diseases, including forms of Parkinson disease and Alzheimer disease. Genetic defects of subunits composing the v-ATPase or v-ATPase-related proteins occur in an increasingly recognized group of familial neurodegenerative diseases. Here, we review the expanding roles of the v-ATPase complex as a platform regulating lysosomal hydrolysis and cellular homeostasis. We discuss the unique vulnerability of neurons to persistent low level lysosomal dysfunction and review recent clinical and experimental studies that link dysfunction of the v-ATPase complex to neurodegenerative diseases across the age spectrum.

mTOR-Mediated Regulation of Dendritic Cell Differentiation and Function

Dendritic cells (DCs) are essential antigen-presenting cells that sample the extra- and intracellular milieu to process antigens for the instruction of T cell responses. The mammalian target of rapamycin (mTOR) network senses environmental cues and is important for numerous cellular processes. This review discusses how DCs use mTOR complexes (mTORC1 and 2) to adapt their cellular metabolism, transcriptional responses, and translation machinery to control DC development, antigen processing, cytokine production, and T cell stimulation. We present a spatiotemporal model suggesting that the mTOR network integrates pattern recognition and growth factor receptor activation with nutritional information from the cell and surrounding tissue to support T cell stimulation and tolerance. mTOR develops into a central player that regulates DC differentiation and immune functions.

Regulation of Vacuolar H+-ATPase (V-ATPase) Reassembly by Glycolysis Flow in 6-Phosphofructo-1-kinase (PFK-1)-deficient Yeast Cells

Yeast 6-phosphofructo-1-kinase (PFK-1) has two subunits, Pfk1p and Pfk2p. Deletion of Pfk2p alters glucose-dependent V-ATPase reassembly and vacuolar acidification (Chan, C. Y., and Parra, K. J. (2014) Yeast phosphofructokinase-1 subunit Pfk2p is necessary for pH homeostasis and glucose-dependent vacuolar ATPase reassembly. J. Biol. Chem. 289, 19448-19457). This study capitalized on the mechanisms suppressing vacuolar H(+)-ATPase (V-ATPase) in pfk2Δ to gain new knowledge of the mechanisms underlying glucose-dependent V-ATPase regulation. Because V-ATPase is fully assembled in pfk2Δ, and glycolysis partially suppressed at steady state, we manipulated glycolysis and assessed its direct involvement on V-ATPase function. At steady state, the ratio of proton transport to ATP hydrolysis increased 24% after increasing the glucose concentration from 2% to 4% to enhance the glycolysis flow in pfk2Δ. Tighter coupling restored vacuolar pH when glucose was abundant and glycolysis operated below capacity. After readdition of glucose to glucose-deprived cells, glucose-dependent V1Vo reassembly was proportional to the glycolysis flow. Readdition of 2% glucose to pfk2Δ cells, which restored 62% of ethanol concentration, led to equivalent 60% V1Vo reassembly levels. Steady-state level of assembly (100% reassembly) was reached at 4% glucose when glycolysis reached a threshold in pfk2Δ (≥40% the wild-type flow). At 4% glucose, the level of Pfk1p co-immunoprecipitated with V-ATPase decreased 58% in pfk2Δ, suggesting that Pfk1p binding to V-ATPase may be inhibitory in the mutant. We concluded that V-ATPase activity at steady state and V-ATPase reassembly after readdition of glucose to glucose-deprived cells are controlled by the glycolysis flow. We propose a new mechanism by which glucose regulates V-ATPase catalytic activity that occurs at steady state without changing V1Vo assembly.

Recent Insights into the Structure, Regulation, and Function of the V-ATPases

The vacuolar (H(+))-ATPases (V-ATPases) are ATP-dependent proton pumps that acidify intracellular compartments and are also present at the plasma membrane. They function in such processes as membrane traffic, protein degradation, virus and toxin entry, bone resorption, pH homeostasis, and tumor cell invasion. V-ATPases are large multisubunit complexes, composed of an ATP-hydrolytic domain (V1) and a proton translocation domain (V0), and operate by a rotary mechanism. This review focuses on recent insights into their structure and mechanism, the mechanisms that regulate V-ATPase activity (particularly regulated assembly and trafficking), and the role of V-ATPases in processes such as cell signaling and cancer. These developments have highlighted the potential of V-ATPases as a therapeutic target in a variety of human diseases.

Regulation of V-ATPase assembly and function of V-ATPases in tumor cell invasiveness

V-ATPases are ATP-driven proton pumps that function within both intracellular compartments and the plasma membrane in a wide array of normal physiological and pathophysiological processes. V-ATPases are composed of a peripheral V(1) domain that hydrolyzes ATP and an integral V(0) domain that transports protons. Regulated assembly of the V-ATPase represents an important mechanism of regulating V-ATPase activity in response to a number of environmental cues. Our laboratory has demonstrated that glucose-dependent assembly of the V-ATPase complex in yeast is controlled by the Ras/cAMP/PKA pathway. By contrast, increased assembly of the V-ATPase during dendritic cell maturation involves the PI-3 kinase and mTORC1 pathways. Recently, we have shown that amino acids regulate V-ATPase assembly in mammalian cells, possibly as a means to maintain adequate levels of amino acids upon nutrient starvation. V-ATPases have also been implicated in cancer cell survival and invasion. V-ATPases are targeted to different cellular membranes by isoforms of subunit a, with a3 targeting V-ATPases to the plasma membrane of osteoclasts. We have shown that highly invasive human breast cancer cell lines express higher levels of the a3 isoform than poorly invasive lines and that knockdown of a3 reduces both expression of V-ATPases at the plasma membrane and in vitro invasion of breast tumor cells. Moreover, overexpression of a3 in a non-invasive breast epithelial line increases both plasma membrane V-ATPases and in vitro invasion. Finally, specific ablation of plasma membrane V-ATPases in highly invasive human breast cancer cells using either an antibody or small molecule approach inhibits both in vitro invasion and migration. These results suggest that plasma membrane and a3-containing V-ATPases represent a novel and important target in the development of therapeutics to limit breast cancer metastasis. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Molecular Mechanisms of Induction of Tolerant and Tolerogenic Intestinal Dendritic Cells in Mice

How does the host manage to tolerate its own intestinal microbiota? A simple question leading to complicated answers. In order to maintain balanced immune responses in the intestine, the host immune system must tolerate commensal bacteria in the gut while it has to simultaneously keep the ability to fight pathogens and to clear infections. If this tender equilibrium is disturbed, severe chronic inflammatory reactions can result. Tolerogenic intestinal dendritic cells fulfil a crucial role in balancing immune responses and therefore creating homeostatic conditions and preventing from uncontrolled inflammation. Although several dendritic cell subsets have already been characterized to play a pivotal role in this process, less is known about definite molecular mechanisms of how intestinal dendritic cells are converted into tolerogenic ones. Here we review how gut commensal bacteria interact with intestinal dendritic cells and why this bacteria-host cell interaction is crucial for induction of dendritic cell tolerance in the intestine. Hereby, different commensal bacteria can have distinct effects on the phenotype of intestinal dendritic cells and these effects are mainly mediated by impacting toll-like receptor signalling in dendritic cells.

Inhibition of mTOR improves the impairment of acidification in autophagic vesicles caused by hepatic steatosis

Recent investigations revealed that dysfunction of autophagy involved in the progression of chronic liver diseases such as alcoholic and nonalcoholic steatohepatitis and hepatocellular neoplasia. Previously, it was reported that hepatic steatosis disturbs autophagic proteolysis via suppression of both autophagic induction and lysosomal function. Here, we demonstrate that autophagic acidification was altered by a decrease in lysosomal proton pump vacuolar-ATPase (V-ATPase) in steatohepatitis. The number of autophagic vesicles was increased in hepatocytes from obese KKAy mice as compared to control. Similarly, autophagic membrane protein LC3-II and lysosomal protein LAMP-2 expression were enhanced in KKAy mice liver. Nevertheless, both phospho-mTOR and p62 expression were augmented in KKAy mice liver. More than 70% of autophagosomes were stained by LysoTracker Red (LTR) in hepatocytes from control mice; however, the percentage of acidic autolysosomes was decreased in hepatocytes from KKAy mice significantly (40.1 ± 3.48%). Both protein and RNA level of V-ATPase subunits ATP6v1a, ATP6v1b, ATP6v1d in isolated lysosomes were suppressed in KKAy mice as compared to control. Interestingly, incubation with mTOR inhibitor rapamycin increased in the rate of LTR-positive autolysosomes in hepatocytes from KKAy mice and suppressed p62 accumulation in the liver from KKAy mice which correlated to an increase in the V-ATPase subunits expression. These results indicate that down-regulation of V-ATPase due to hepatic steatosis causes autophagic dysfunction via disruption of lysosomal and autophagic acidification. Moreover, activation of mTOR plays a pivotal role on dysregulation of lysosomal and autophagic acidification by modulation of V-ATPase expression and could therefore be a useful therapeutic target to ameliorate dysfunction of autophagy in NAFLD.

Amino Acid Availability Modulates Vacuolar H+-ATPase Assembly

The vacuolar H(+)-ATPase (V-ATPase) is an ATP-dependent proton pump composed of a peripheral ATPase domain (V1) and a membrane-integral proton-translocating domain (V0) and is involved in many normal and disease processes. An important mechanism of regulating V-ATPase activity is reversible assembly of the V1 and V0 domains. Increased assembly in mammalian cells occurs under various conditions and has been shown to involve PI3K. The V-ATPase is necessary for amino acid-induced activation of mechanistic target of rapamycin complex 1 (mTORC1), which is important in controlling cell growth in response to nutrient availability and growth signals. The V-ATPase undergoes amino acid-dependent interactions with the Ragulator complex, which is involved in recruitment of mTORC1 to the lysosomal membrane during amino acid sensing. We hypothesized that changes in the V-ATPase/Ragulator interaction might involve amino acid-dependent changes in V-ATPase assembly. To test this, we measured V-ATPase assembly by cell fractionation in HEK293T cells treated with and without amino acids. V-ATPase assembly increases upon amino acid starvation, and this effect is reversed upon readdition of amino acids. Lysosomes from amino acid-starved cells possess greater V-ATPase-dependent proton transport, indicating that assembled pumps are catalytically active. Amino acid-dependent changes in both V-ATPase assembly and activity are independent of PI3K and mTORC1 activity, indicating the involvement of signaling pathways distinct from those implicated previously in controlling assembly. By contrast, lysosomal neutralization blocks the amino acid-dependent change in assembly and reactivation of mTORC1 after amino acid starvation. These results identify an important new stimulus for controlling V-ATPase assembly.

Cross-Presentation of Cell-Associated Antigens by MHC Class I in Dendritic Cell Subsets

Dendritic cells (DCs) have the unique ability to pick up dead cells carrying antigens in tissue and migrate to the lymph nodes where they can cross-present cell-associated antigens by MHC class I to CD8⁺ T cells. There is strong in vivo evidence that the mouse XCR1⁺ DCs subset acts as a key player in this process. The intracellular processes underlying cross-presentation remain controversial and several pathways have been proposed. Indeed, a wide number of studies have addressed the cellular process of cross-presentation in vitro using a variety of sources of antigen and antigen-presenting cells. Here, we review the in vivo and in vitro evidence supporting the current mechanistic models and disscuss their physiological relevance to the cross-presentation of cell-associated antigens by DCs subsets.