Biochemical and biophysical properties of interactions between subunits of the peripheral stalk region of human V-ATPase

Wnt signaling in cancer: therapeutic targeting of Wnt signaling beyond β-catenin and the destruction complex

Wnt/β-catenin signaling is implicated in many physiological processes, including development, tissue homeostasis, and tissue regeneration. In human cancers, Wnt/β-catenin signaling is highly activated, which has led to the development of various Wnt signaling inhibitors for cancer therapies. Nonetheless, the blockade of Wnt signaling causes side effects such as impairment of tissue homeostasis and regeneration. Recently, several studies have identified cancer-specific Wnt signaling regulators. In this review, we discuss the Wnt inhibitors currently being used in clinical trials and suggest how additional cancer-specific regulators could be utilized to treat Wnt signaling-associated cancer.

Vacuolar proton-translocating ATPase is required for antifungal resistance and virulence of Candida glabrata

Vacuolar proton-translocating ATPase (V-ATPase) is located in fungal vacuolar membranes. It is involved in multiple cellular processes, including the maintenance of intracellular ion homeostasis by maintaining acidic pH within the cell. The importance of V-ATPase in virulence has been demonstrated in several pathogenic fungi, including Candida albicans. However, it remains to be determined in the clinically important fungal pathogen Candida glabrata. Increasing multidrug resistance of C. glabrata is becoming a critical issue in the clinical setting. In the current study, we demonstrated that the plecomacrolide V-ATPase inhibitor bafilomycin B₁ exerts a synergistic effect with azole antifungal agents, including fluconazole and voriconazole, against a C. glabrata wild-type strain. Furthermore, the deletion of the VPH2 gene encoding an assembly factor of V-ATPase was sufficient to interfere with V-ATPase function in C. glabrata, resulting in impaired pH homeostasis in the vacuole and increased sensitivity to a variety of environmental stresses, such as alkaline conditions (pH 7.4), ion stress (Na⁺, Ca²⁺, Mn²⁺, and Zn²⁺ stress), exposure to the calcineurin inhibitor FK506 and antifungal agents (azoles and amphotericin B), and iron limitation. In addition, virulence of C. glabrata Δvph2 mutant in a mouse model of disseminated candidiasis was reduced in comparison with that of the wild-type and VPH2-reconstituted strains. These findings support the notion that V-ATPase is a potential attractive target for the development of effective antifungal strategies.

Vacuolar-type ATPase: A proton pump to lysosomal trafficking

Vacuolar-type ATPase (V-ATPase), initially identified in yeast and plant vacuoles, pumps protons into the lumen of organelles coupled with ATP hydrolysis. The mammalian counterpart is found ubiquitously in endomembrane organelles and the plasma membrane of specialized cells such as osteoclasts. V-ATPase is also present in unique organelles such as insulin secretory granules, neural synaptic vesicles, and acrosomes of spermatozoa. Consistent with its diverse physiological roles and unique localization, the seven subunits of V-ATPase have 2-4 isoforms that are organelle- or cell-specific. Subunits of the enzyme function in trafficking organelles and vesicles by interacting with small molecule GTPases. During osteoclast differentiation, one of the four isoforms of subunit a, a3, is indispensable for secretory lysosome trafficking to the plasma membrane. Diseases such as osteopetrosis, renal acidosis, and hearing loss are related to V-ATPase isoforms. In addition to its role as an enzyme, V-ATPase has versatile physiological roles in eukaryotic cells.

Breaking up and making up: The secret life of the vacuolar H+ -ATPase

The vacuolar ATPase (V-ATPase; V₁ V_o -ATPase) is a large multisubunit proton pump found in the endomembrane system of all eukaryotic cells where it acidifies the lumen of subcellular organelles including lysosomes, endosomes, the Golgi apparatus, and clathrin-coated vesicles. V-ATPase function is essential for pH and ion homeostasis, protein trafficking, endocytosis, mechanistic target of rapamycin (mTOR), and Notch signaling, as well as hormone secretion and neurotransmitter release. V-ATPase can also be found in the plasma membrane of polarized animal cells where its proton pumping function is involved in bone remodeling, urine acidification, and sperm maturation. Aberrant (hypo or hyper) activity has been associated with numerous human diseases and the V-ATPase has therefore been recognized as a potential drug target. Recent progress with moderate to high-resolution structure determination by cryo electron microscopy and X-ray crystallography together with sophisticated single-molecule and biochemical experiments have provided a detailed picture of the structure and unique mode of regulation of the V-ATPase. This review summarizes the recent advances, focusing on the structural and biophysical aspects of the field.

Binding interactions of the peripheral stalk subunit isoforms from human V-ATPase

The mammalian peripheral stalk subunits of the vacuolar-type H(+)-ATPases (V-ATPases) possess several isoforms (C1, C2, E1, E2, G1, G2, G3, a1, a2, a3, and a4), which may play significant role in regulating ATPase assembly and disassembly in different tissues. To better understand the structure and function of V-ATPase, we expressed and purified several isoforms of the human V-ATPase peripheral stalk: E1G1, E1G2, E1G3, E2G1, E2G2, E2G3, C1, C2, H, a1NT, and a2NT. Here, we investigated and characterized the isoforms of the peripheral stalk region of human V-ATPase with respect to their affinity and kinetics in different combination. We found that different isoforms interacted in a similar manner with the isoforms of other subunits. The differences in binding affinities among isoforms were minor from our in vitro studies. However, such minor differences from the binding interaction among isoforms might provide valuable information for the future structural-functional studies of this holoenzyme.

Inflammation and macrophage modulation in adipose tissues

The adipose tissue is an active endocrine organ that harbours not only mature and developing adipocytes but also a wide array of immune cells, including macrophages, a key immune cell in determining metabolic functionality. With adipose tissue expansion, M1 pro-inflammatory macrophage infiltration increases, activates other immune cells, and affects lipid trafficking and metabolism, in part via inhibiting mitochondrial function and increasing reactive oxygen species (ROS). The pro-inflammatory cytokines produced and released interfere with insulin signalling, while inhibiting M1 macrophage activation improves systemic insulin sensitivity. In healthy adipose tissue, M2 alternative macrophages predominate and associate with enhanced lipid handling and mitochondrial function, anti-inflammatory cytokine production, and inhibition of ROS. The sequence of events leading to macrophage infiltration and activation in adipose tissue remains incompletely understood but lipid handling of both macrophages and adipocytes appears to play a major role.

The signaling lipid PI(3,5)P₂ stabilizes V₁-V(o) sector interactions and activates the V-ATPase

Vacuolar proton-translocating ATPases (V-ATPases) are highly conserved, ATP-driven proton pumps regulated by reversible dissociation of its cytosolic, peripheral V1 domain from the integral membrane V(o) domain. Multiple stresses induce changes in V1-V(o) assembly, but the signaling mechanisms behind these changes are not understood. Here we show that certain stress-responsive changes in V-ATPase activity and assembly require the signaling lipid phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2). V-ATPase activation through V1-V(o) assembly in response to salt stress is strongly dependent on PI(3,5)P2 synthesis. Purified V(o) complexes preferentially bind to PI(3,5)P2 on lipid arrays, suggesting direct binding between the lipid and the membrane sector of the V-ATPase. Increasing PI(3,5)P2 levels in vivo recruits the N-terminal domain of V(o)-sector subunit Vph1p from cytosol to membranes, independent of other subunits. This Vph1p domain is critical for V1-V(o) interaction, suggesting that interaction of Vph1p with PI(3,5)P2-containing membranes stabilizes V1-V(o) assembly and thus increases V-ATPase activity. These results help explain the previously described vacuolar acidification defect in yeast fab1 and vac14 mutants and suggest that human disease phenotypes associated with PI(3,5)P2 loss may arise from compromised V-ATPase stability and regulation.

Eukaryotic V-ATPase: novel structural findings and functional insights

The eukaryotic V-type adenosine triphosphatase (V-ATPase) is a multi-subunit membrane protein complex that is evolutionarily related to F-type adenosine triphosphate (ATP) synthases and A-ATP synthases. These ATPases/ATP synthases are functionally conserved and operate as rotary proton-pumping nano-motors, invented by Nature billions of years ago. In the first part of this review we will focus on recent structural findings of eukaryotic V-ATPases and discuss the role of different subunits in the function of the V-ATPase holocomplex. Despite structural and functional similarities between rotary ATPases, the eukaryotic V-ATPases are the most complex enzymes that have acquired some unconventional cellular functions during evolution. In particular, the novel roles of V-ATPases in the regulation of cellular receptors and their trafficking via endocytotic and exocytotic pathways were recently uncovered. In the second part of this review we will discuss these unique roles of V-ATPases in modulation of function of cellular receptors, involved in the development and progression of diseases such as cancer and diabetes as well as neurodegenerative and kidney disorders. Moreover, it was recently revealed that the V-ATPase itself functions as an evolutionarily conserved pH sensor and receptor for cytohesin-2/Arf-family GTP-binding proteins. Thus, in the third part of the review we will evaluate the structural basis for and functional insights into this novel concept, followed by the analysis of the potentially essential role of V-ATPase in the regulation of this signaling pathway in health and disease. Finally, future prospects for structural and functional studies of the eukaryotic V-ATPase will be discussed.

Advances in targeting the vacuolar proton-translocating ATPase (V-ATPase) for anti-fungal therapy

Vacuolar proton-translocating ATPase (V-ATPase) is a membrane-bound, multi-subunit enzyme that uses the energy of ATP hydrolysis to pump protons across membranes. V-ATPase activity is critical for pH homeostasis and organelle acidification as well as for generation of the membrane potential that drives secondary transporters and cellular metabolism. V-ATPase is highly conserved across species and is best characterized in the model fungus Saccharomyces cerevisiae. However, recent studies in mammals have identified significant alterations from fungi, particularly in the isoform composition of the 14 subunits and in the regulation of complex disassembly. These differences could be exploited for selectivity between fungi and humans and highlight the potential for V-ATPase as an anti-fungal drug target. Candida albicans is a major human fungal pathogen and causes fatality in 35% of systemic infections, even with anti-fungal treatment. The pathogenicity of C. albicans correlates with environmental, vacuolar, and cytoplasmic pH regulation, and V-ATPase appears to play a fundamental role in each of these processes. Genetic loss of V-ATPase in pathogenic fungi leads to defective virulence, and a comprehensive picture of the mechanisms involved is emerging. Recent studies have explored the practical utility of V-ATPase as an anti-fungal drug target in C. albicans, including pharmacological inhibition, azole therapy, and targeting of downstream pathways. This overview will discuss these studies as well as hypothetical ways to target V-ATPase and novel high-throughput methods for use in future drug discovery screens.