Current Protein and Peptide Science - Volume 13, Issue 2, 2012

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Vacuolar ATPases (V-ATPases) are large multisubunit complexes that actively transport protons across cellular membranes to acidify intracellular compartments, thereby serving a critical housekeeping function. In addition, VATPases are also expressed on the plasma membrane of cell types such as kidney epithelia and osteoclasts, which require high levels of proton secretion to perform their specialized activities. This multiplicity of function is achieved by the expression of numerous V-ATPase subunit isoforms that are mixed and matched to produce complexes required for each cellular activity. Multiple regulatory mechanisms are necessary to allow coordinated expression of V-ATPase subunit proteins involved in both housekeeping and specialized functions. This review will summarize studies during the last two decades that have revealed transcriptional and post-transcriptional controls that govern expression of V-ATPase subunits. These studies are beginning to elucidate overarching mechanisms that permit coordinated expression of ubiquitous subunits while directing tissue-specific expression of others.

Vacuolar proton-translocating ATPases (V-ATPases) are highly conserved proton pumps consisting of a peripheral membrane subcomplex called V1, which contains the sites of ATP hydrolysis, attached to an integral membrane subcomplex called Vo, which encompasses the proton pore. V-ATPase regulation by reversible dissociation, characterized by release of assembled V1 sectors into the cytosol and inhibition of both ATPase and proton transport activities, was first identified in tobacco hornworm and yeast. It has since become clear that modulation of V-ATPase assembly level is also a regulatory mechanism in mammalian cells. In this review, the implications of reversible disassembly for V-ATPase structure are discussed, along with insights into underlying subunit-subunit interactions provided by recent structural work. Although initial experiments focused on glucose deprivation as a trigger for disassembly, it is now clear that V-ATPase assembly can be regulated by other extracellular conditions. Consistent with a complex, integrated response to extracellular signals, a number of different regulatory proteins, including RAVE/rabconnectin, aldolase and other glycolytic enzymes, and protein kinase A have been suggested to control V-ATPase assembly and disassembly. It is likely that multiple signaling pathways dictate the ultimate level of assembly and activity. Tissue-specific V-ATPase inhibition is a potential therapy for osteoporosis and cancer; the possibility of exploiting reversible disassembly in design of novel V-ATPase inhibitors is discussed.

The vacuolar (H+)-ATPase (V-ATPase) is a universal proton pump and its activity is required for a variety of cell-biological processes such as membrane trafficking, receptor-mediated endocytosis, lysosomal protein degradation, osteoclastic bone resorption and maintenance of acid-base homeostasis by renal intercalated cells. In neuronal and neuroendocrine cells, the V-ATPase is the major regulator of intragranular acidification which is indispensable for correct prohormone processing and neurotransmitter uptake. In these specialized cells, the V-ATPase is equipped with the accessory subunits ATP6AP1/Ac45 and ATP6AP2/(pro) renin receptor. Recent studies have shown that Ac45 interacts with the V0- sector of the V-ATPase complex, thereby regulating the intragranular pH and Ca2+-dependent exocytotic membrane fusion. Thus, Ac45 can be considered as a V-ATPase regulator in the neuroendocrine secretory pathway. ATP6AP2 has recently been found to be identical to the (pro) renin receptor and has a dual role: (i) in the renin-angiotensin system that also regulates V-ATPase activity; (ii) acting as an adapter by binding to both the V-ATPase and the Wnt receptor complex, thereby recruiting the receptor complex into an acidic microenvironment. We here provide an overview of the two V-ATPase accessory subunits as novel key players in V-ATPase regulation. We argue that the accessory subunits are candidate genes for V-ATPase-related human disorders and promising targets for manipulating V-ATPase functioning in vivo.

The ubiquitous and essential V-ATPase is a worthy chemotherapeutic target in the escalating battle against invasive fungal infections. Pathogenic fungi require optimum V-ATPase function for secretion of virulence factors, induction of stress response pathways, hyphal morphology and homeostasis of pH and other cations in order to successfully survive within and colonize the host. This review discusses why impairment of V-ATPase activity confers multidrug sensitivity and loss of virulence. Recent evidence points to the V-ATPase as a novel downstream target of the azole class of antifungals that inhibit the biogenesis of ergosterol. Depletion of ergosterol from vacuolar membranes led to progressive alkalization of yeast vacuoles, loss of V-ATPase activity and growth inhibition that could be rescued by exogenous ergosterol feeding. Other studies point to a critical role for sphingolipids, phospholipids and cardiolipin in V-ATPase function. Thus, drugs that inhibit the V-ATPase directly, or indirectly by modulating the membrane milieu, can profoundly affect fungal viability and virulence. These findings justify a systematic screen for fungal specific V-ATPase inhibitors or membrane active compounds that can be used in antifungal chemotherapy.

The unique ability of the osteoclasts to resorb the calcified bone matrix is dependent on secretion of hydrochloric acid. This process is mediated by a vacuolar H+ ATPase (V-ATPase) and a chloride-proton antiporter. The structural subunit of the V-ATPase, a3, is highly specific for osteoclasts, and mutations in a3 lead to infantile malignant osteopetrosis, a phenomenon characterized by increased bone mass, an increased number of non-resorbing osteoclasts, and a complete lack of bone resorption. Importantly, these individuals have normal or even increased osteoblast numbers and bone formation suggesting that the osteoclasts, but not their resorptive capability, relay an anabolic signal, and, hence, that bone formation can be uncoupled from bone resorption when the a3 subunit is eliminated by mutations, or possibly by pharmacological intervention. The pharmacological profile of the a3 subunit as a highly specific target with a mode of action profile augmenting uncoupling and sustained bone formation, as derived from osteopetrotic patients and mice, highlights the relevance of the V-ATPase in future osteoporosis drug development. However, as illustrated by numerous attempts at developing specific inhibitors of the osteoclastic V-ATPase it is a very difficult target to work with, and an inhibitor possessing the desired profile remains elusive, although highly promising approaches recently have been launched.

Up-regulated aerobic glycolysis is a hallmark of malignant cancers. Little is understood about the reasons why malignant tumors up-regulate glycolysis and acidify their microenvironment. Signaling pathways involved in glucose changes are numerous. However, the identity of the internal glucose signal remains obscure. In this review we address the question of the significance of vacuolar proton ATPase (V-ATPase) and its relationship to up-regulated glycolysis in tumors. We know that glycolysis is extremely sensitive to changes in pH. Importantly, the V-ATPase activity is sensitive to glucose availability. Therefore, we propose that pH acts as the glucose signal via the V-ATPase that responds to changes in intracellular pH and acts as a sensor. We hypothesize that the increase in glycolysis leads to intracellular acidification and activates the V-ATPase to maintain a more alkaline intracellular pH in tumors by up-regulating glycolysis. This review attempts to provide a comprehensive description of the current knowledge about the role of V-ATPase in cancer, highlighting its role as a key player in the pH signaling pathway.

Over the last three decades, V-ATPases have emerged from the obscurity of poorly understood membrane proton transport phenomena to being recognized as ubiquitous proton pumps that underlie vital cellular processes in all eukaryotic and many prokaryotic cells. These exquisitely complex molecular motors also engage in diverse specialized roles contributing to development, tissue function and pH homeostasis within complex organisms. Increasingly, mutations and misappropriation of V-ATPase function have been linked to diseases, ranging from sclerosing bone pathologies and renal tubular acidosis to bone-loss disorders and cancer metastasis. Much remains to be learned about the details of V-ATPase cell and molecular biology; nevertheless, interest in V-ATPases as potential therapeutic targets has burgeoned in recent years. In this review, we present a history of our involvement and contributions to the understanding of V-ATPase structure and function and our nascent and ongoing contributions to translating the knowledge gained from basic research on the nature of V-ATPases into tools for drug discovery. We focus here primarily on the treatment of bone-loss pathologies, like osteoporosis, and present proof-of-concept for a drug screening strategy based on targeting a3-B2 subunit interactions within the V-ATPase complex.

Binding between vacuolar H+-ATPases (V-ATPases) and microfilaments is mediated by an actin binding domain in the B-subunit. Both isoforms of mammalian B-subunit bind microfilaments with high affinity. A similar actinbinding activity has been demonstrated in the B-subunit of yeast. A conserved “profilin-like” domain in the B-subunit mediates this actin-binding activity, named due to its sequence and structural similarity to an actin-binding surface of the canonical actin binding protein profilin. Subtle mutations in the “profilin-like” domain eliminate actin binding activity without disrupting the ability of the altered protein to associate with the other subunits of V-ATPase to form a functional proton pump. Analysis of these mutated B-subunits suggests that the actin-binding activity is not required for the “housekeeping” functions of V-ATPases, but is important for certain specialized roles. In osteoclasts, the actin-binding activity is required for transport of V-ATPases to the plasma membrane, a prerequisite for bone resorption. A virtual screen led to the identification of enoxacin as a small molecule that bound to the actin-binding surface of the B2-subunit and competitively inhibited B2-subunit and actin interaction. Enoxacin disrupted osteoclastic bone resorption in vitro, but did not affect osteoblast formation or mineralization. Recently, enoxacin was identified as an inhibitor of the virulence of Candida albicans and more importantly of cancer growth and metastasis. Efforts are underway to determine the mechanisms by which enoxacin and other small molecule inhibitors of B2 and microfilament binding interaction selectively block bone resorption, the virulence of Candida, cancer growth, and metastasis.