NTPDase1 and NTPDase2 immunolocalization in mouse cochlea: implications for regulation of p2 receptor signaling

Purinergic Signalling in the Cochlea

The mammalian cochlea is the sensory organ of hearing with a delicate, highly organised structure that supports unique operating mechanisms. ATP release from the secretory tissues of the cochlear lateral wall (stria vascularis) triggers numerous physiological responses by activating P2 receptors in sensory, supporting and neural tissues. Two families of P2 receptors, ATP-gated ion channels (P2X receptors) and G protein-coupled P2Y receptors, activate intracellular signalling pathways that regulate cochlear development, homeostasis, sensory transduction, auditory neurotransmission and response to stress. Of particular interest is a purinergic hearing adaptation, which reflects the critical role of the P2X₂ receptor in adaptive cochlear response to elevated sound levels. Other P2 receptors are involved in the maturation of neural processes and frequency selectivity refinement in the developing cochlea. Extracellular ATP signalling is regulated by a family of surface-located enzymes collectively known as "ectonucleotidases" that hydrolyse ATP to adenosine. Adenosine is a constitutive cell metabolite with an established role in tissue protection and regeneration. The differential activation of A₁ and A_2A adenosine receptors defines the cochlear response to injury caused by oxidative stress, inflammation, and activation of apoptotic pathways. A₁ receptor agonism, A_2A receptor antagonism, and increasing adenosine levels in cochlear fluids all represent promising therapeutic tools for cochlear rescue from injury and prevention of hearing loss.

Purinergic signaling in cochlear supporting cells reduces hair cell excitability by increasing the extracellular space

Neurons in developing sensory pathways exhibit spontaneous bursts of electrical activity that are critical for survival, maturation and circuit refinement. In the auditory system, intrinsically generated activity arises within the cochlea, but the molecular mechanisms that initiate this activity remain poorly understood. We show that burst firing of mouse inner hair cells prior to hearing onset requires P2RY1 autoreceptors expressed by inner supporting cells. P2RY1 activation triggers K+ efflux and depolarization of hair cells, as well as osmotic shrinkage of supporting cells that dramatically increased the extracellular space and speed of K+ redistribution. Pharmacological inhibition or genetic disruption of P2RY1 suppressed neuronal burst firing by reducing K+ release, but unexpectedly enhanced their tonic firing, as water resorption by supporting cells reduced the extracellular space, leading to K+ accumulation. These studies indicate that purinergic signaling in supporting cells regulates hair cell excitability by controlling the volume of the extracellular space.

Purinergic Signaling and Cochlear Injury-Targeting the Immune System?

Hearing impairment is the most common sensory deficit, affecting more than 400 million people worldwide. Sensorineural hearing losses currently lack any specific or efficient pharmacotherapy largely due to the insufficient knowledge of the pathomechanism. Purinergic signaling plays a substantial role in cochlear (patho)physiology. P2 (ionotropic P2X and the metabotropic P2Y) as well as adenosine receptors expressed on cochlear sensory and non-sensory cells are involved mostly in protective mechanisms of the cochlea. They are implicated in the sensitivity adjustment of the receptor cells by a K⁺ shunt and can attenuate the cochlear amplification by modifying cochlear micromechanics. Cochlear blood flow is also regulated by purines. Here, we propose to comprehend this field with the purine-immune interactions in the cochlea. The role of harmful immune mechanisms in sensorineural hearing losses has been emerging in the horizon of cochlear pathologies. In addition to decreasing hearing sensitivity and increasing cochlear blood supply, influencing the immune system can be the additional avenue for pharmacological targeting of purinergic signaling in the cochlea. Elucidating this complexity of purinergic effects on cochlear functions is necessary and it can result in development of new therapeutic approaches in hearing disabilities, especially in the noise-induced ones.

Purinergic Signaling and Aminoglycoside Ototoxicity: The Opposing Roles of P1 (Adenosine) and P2 (ATP) Receptors on Cochlear Hair Cell Survival

Purinergic signaling regulates important physiological processes and the homeostatic response to stress in the cochlea via extracellular nucleosides (adenosine) and nucleotides (ATP, UTP). Using a previously established organotypic culture model, the current study investigated the effect of purinergic P1 (adenosine) and P2 (ATP) receptor activation on the survival of the sensory hair cell population in the cochlea exposed to the ototoxic aminoglycoside neomycin. Organ of Corti explants were obtained from C57BL/6 mice at postnatal day 3 (P3) and maintained in normal culture medium (with or without purine receptor agonists or analogs) for 19.5 h prior to neomycin exposure (1 mM, 3 h) followed by a further incubation for 19.5 h in culture medium. The cochlear explants were then fixed in 4% paraformaldehyde (PFA) and sensory hair cells labeled with Alexa 488-phalloidin. Neomycin induced a substantial loss of the sensory hair cells, mostly in the middle segment of the cochlea. This neomycin-induced ototoxicity was unaffected by the addition of P2 receptor agonists (ATP and UTP) in the culture medium, whilst the addition of their slowly-hydrolyzable analogs (ATPγS, UTPγS) aggravated neomycin-induced sensory hair cell loss. In contrast, the activation of P1 receptors by adenosine or adenosine amine congener (ADAC) conferred partial protection from neomycin ototoxicity. This study demonstrates a pro-survival effect of P1 receptor stimulation, whilst prolonged activation of P2 receptors has an opposite effect. Based on these findings, we postulate that P1 and P2 receptors orchestrate differential responses to cochlear injury and that the balance of these receptors is important for maintaining cochlear homeostasis following ototoxic injury.

Characterization of ectonucleotidase expression in the rat carotid body: regulation by chronic hypoxia

The carotid body (CB) chemoreflex maintains blood Po₂ and Pco₂/H⁺ homeostasis and displays sensory plasticity during exposure to chronic hypoxia. Purinergic signaling via P1 and P2 receptors plays a pivotal role in shaping the afferent discharge at the sensory synapse containing catecholaminergic chemoreceptor (type I) cells, glial-like type II cells, and sensory (petrosal) nerve endings. However, little is known about the family of ectonucleotidases that control synaptic nucleotide levels. Using quantitative PCR (qPCR), we first compared expression levels of ectonucleoside triphosphate diphosphohydrolases (NTPDases1,2,3,5,6) and ecto-5'-nucleotidase (E5'Nt/CD73) mRNAs in juvenile rat CB vs. brain, petrosal ganglia, sympathetic (superior cervical) ganglia, and a sympathoadrenal chromaffin (MAH) cell line. In whole CB extracts, qPCR revealed a high relative expression of surface-located members NTPDase1,2 and E5'Nt/CD73, compared with low NTPDase3 expression. Immunofluorescence staining of CB sections or dissociated CB cultures localized NTPDase2,3 and E5'Nt/CD73 protein to the periphery of type I clusters, and in association with sensory nerve fibers and/or isolated type II cells. Interestingly, in CBs obtained from rats reared under chronic hypobaric hypoxia (~60 kPa, equivalent to 4,300 m) for 5-7 days, in addition to the expected upregulation of tyrosine hydroxylase and VEGF mRNAs, there was a significant upregulation of NTPDase3 and E5'Nt/CD73 mRNA, but a downregulation of NTPDase1 and NTPDase2 relative to normoxic controls. We conclude that NTPDase1,2,3 and E5'Nt/CD73 are the predominant surface-located ectonucleotidases in the rat CB and suggest that their differential regulation during chronic hypoxia may contribute to CB plasticity via control of synaptic ATP, ADP, and adenosine pools.

NTPDase2+ Cells Generate Lingual Epithelia and Papillae

The tongue epithelium is one of the most rapidly self-renewing tissues in adult mammals. Multiple stem cell populations are currently believed to exist in tongue epithelia. Keratin 14 (K14) positive cells differentiate into either lingual epithelia or lingual papillae, while ecto-nucleoside triphosphate diphosphohydrolase 2 (NTPDase2) is associated with neural stem cells and astrocyte-like cells ensheathing the migrating neuroblasts. Here, using a transgenic mouse expressing rtTA from the mouse NTPDase2 promoter, we generated an inducible model by treatment with Doxycycline. By immunohistochemical analysis and in situ hybridization, we found exclusive expression of NTPDase2 in lingual epithelia and lingual papillae. Using inducible genetic cell fate mapping, we further showed that the NTPDase2+ cells generated lingual papillae and epithelia in the adult tongue. Finally, building on our previously proposed paradigm of cell migration stream, a model is further described here for lingual epithelia cell genesis. In short, the current results not only extend our understanding of the cell migration stream in lingual epithelia and lingual papillae, but they also support the concept of multiple stem cell populations in lingual epithelia and papillae.

Role of adenosine kinase in cochlear development and response to noise

Adenosine signalling has an important role in cochlear protection from oxidative stress. In most tissues, intracellular adenosine kinase (ADK) is the primary route of adenosine metabolism and the key regulator of intracellular and extracellular adenosine levels. The present study provides the first evidence for ADK distribution in the adult and developing rat cochlea. In the adult cochlea, ADK was localized to the nuclear or perinuclear region of spiral ganglion neurons, lateral wall tissues, and epithelial cells lining scala media. In the developing cochlea, ADK was strongly expressed in multiple cell types at birth and reached its peak level of expression at postnatal day 21 (P21). Ontogenetic changes in ADK expression were evident in the spiral ganglion, organ of Corti, and stria vascularis. In the spiral ganglion, ADK showed a shift from predominantly satellite cell immunolabelling at P1 to neuronal expression from P14 onward. In contrast to the role of ADK in various aspects of cochlear development, the ADK contribution to the cochlear response to noise stress was less obvious. Transcript and protein levels of ADK were unaltered in the cochlea exposed to broadband noise (90-110 dBSPL, 24 hr), and the selective inhibition of ADK in the cochlea with ABT-702 failed to restore hearing thresholds after exposure to traumatic noise. This study indicates that ADK is involved in purine salvage pathways for nucleotide synthesis in the adult cochlea, but its role in the regulation of adenosine signalling under physiological and pathological conditions has yet to be established.

Adenosine and the auditory system

Adenosine is a signalling molecule that modulates cellular activity in the central nervous system and peripheral organs via four G protein-coupled receptors designated A₁, A_2A, A_2B, and A₃. This review surveys the literature on the role of adenosine in auditory function, particularly cochlear function and its protection from oxidative stress. The specific tissue distribution of adenosine receptors in the mammalian cochlea implicates adenosine signalling in sensory transduction and auditory neurotransmission although functional studies have demonstrated that adenosine stimulates cochlear blood flow, but does not alter the resting and sound-evoked auditory potentials. An interest in a potential otoprotective role for adenosine has recently evolved, fuelled by the capacity of A₁ adenosine receptors to prevent cochlear injury caused by acoustic trauma and ototoxic drugs. The balance between A₁ and A_2A receptors is conceived as critical for cochlear response to oxidative stress, which is an underlying mechanism of the most common inner ear pathologies (e.g. noise-induced and age-related hearing loss, drug ototoxicity). Enzymes involved in adenosine metabolism, adenosine kinase and adenosine deaminase, are also emerging as attractive targets for controlling oxidative stress in the cochlea. Other possible targets include ectonucleotidases that generate adenosine from extracellular ATP, and nucleoside transporters, which regulate adenosine concentrations on both sides of the plasma membrane. Developments of selective adenosine receptor agonists and antagonists that can cross the blood-cochlea barrier are bolstering efforts to develop therapeutic interventions aimed at ameliorating cochlear injury. Manipulations of the adenosine signalling system thus hold significant promise in the therapeutic management of oxidative stress in the cochlea.

Differential expression of P2Y receptors in the rat cochlea during development

Purinergic signaling has broad physiological significance to the hearing organ, involving signal transduction via ionotropic P2X receptors and metabotropic G-protein-coupled P2Y and P1 (adenosine), alongside conversion of nucleotides and nucleosides by ecto-nucleotidases and ecto-nucleoside diphosphokinase. In addition, ATP release is modulated by acoustic overstimulation or stress and involves feedback regulation. Many of these principal elements of the purinergic signaling complex have been well characterized in the cochlea, while the characterization of P2Y receptor expression is emerging. The present study used immunohistochemistry to evaluate the expression of five P2Y receptors, P2Y(1), P2Y(2), P2Y(4), P2Y(6), and P2Y(12), during development of the rat cochlea. Commencing in the late embryonic period, the P2Y receptors studied were found in the cells lining the cochlear partition, associated with establishment of the electrochemical environment which provides the driving force for sound transduction. In addition, early postnatal P2Y(2) and P2Y(4) protein expression in the greater epithelial ridge, part of the developing hearing organ, supports the view that initiation and regulation of spontaneous activity in the hair cells prior to hearing onset is mediated by purinergic signaling. Sub-cellular compartmentalization of P2Y receptor expression in sensory hair cells, and diversity of receptor expression in the spiral ganglion neurons and their satellite cells, indicates roles for P2Y receptor-mediated Ca(2+)-signaling in sound transduction and auditory neuron excitability. Overall, the dynamics of P2Y receptor expression during development of the cochlea complement the other elements of the purinergic signaling complex and reinforce the significance of extracellular nucleotide and nucleoside signaling to hearing.

ATP-induced morphological changes in supporting cells of the developing cochlea

The developing cochlea of mammals contains a large group of columnar-shaped cells, which together form a structure known as Kölliker's organ. Prior to the onset of hearing, these inner supporting cells periodically release adenosine 5'-triphosphate (ATP), which activates purinergic receptors in surrounding supporting cells, inner hair cells and the dendrites of primary auditory neurons. Recent studies indicate that purinergic signaling between inner supporting cells and inner hair cells initiates bursts of action potentials in auditory nerve fibers before the onset of hearing. ATP also induces prominent effects in inner supporting cells, including an increase in membrane conductance, a rise in intracellular Ca(2+), and dramatic changes in cell shape, although the importance of ATP signaling in non-sensory cells of the developing cochlea remains unknown. Here, we review current knowledge pertaining to purinergic signaling in supporting cells of Kölliker's organ and focus on the mechanisms by which ATP induces changes in their morphology. We show that these changes in cell shape are preceded by increases in cytoplasmic Ca(2+), and provide new evidence indicating that elevation of intracellular Ca(2+) and IP(3) are sufficient to initiate shape changes. In addition, we discuss the possibility that these ATP-mediated morphological changes reflect crenation following the activation of Ca(2+)-activated Cl(-) channels, and speculate about the possible functions of these changes in cell morphology for maturation of the cochlea.

Purinergic signaling in cochleovestibular hair cells and afferent neurons

Purinergic signaling in the mammalian cochleovestibular hair cells and afferent neurons is reviewed. The scope includes P2 and P1 receptors in the inner hair cells (IHCs) of the cochlea, the type I spiral ganglion neurons (SGNs) that convey auditory signals from IHCs, the vestibular hair cells (VHCs) in the vestibular end organs (macula in the otolith organs and crista in the semicircular canals), and the vestibular ganglion neurons (VGNs) that transmit postural and rotatory information from VHCs. Various subtypes of P2X ionotropic receptors are expressed in IHCs as well as P2Y metabotropic receptors that mobilize intracellular calcium. Their functional roles still remain speculative, but adenosine 5'-triphosphate (ATP) could regulate the spontaneous activity of the hair cells during development and the receptor potentials of mature hair cells during sound stimulation. In SGNs, P2Y metabotropic receptors activate a nonspecific cation conductance that is permeable to large cations as NMDG(+) and TEA(+). Remarkably, this depolarizing nonspecific conductance in SGNs can also be activated by other metabotropic processes evoked by acetylcholine and tachykinin. The molecular nature and the role of this depolarizing channel are unknown, but its electrophysiological properties suggest that it could lie within the transient receptor potential channel family and could regulate the firing properties of the afferent neurons. Studies on the vestibular partition (VHC and VGN) are sparse but have also shown the expression of P2X and P2Y receptors. There is still little evidence of functional P1 (adenosine) receptors in the afferent system of the inner ear.

Immunocytochemical localization of NTPDases1 and 2 in the neural retina of mouse and zebrafish

Ectonucleoside triphosphate diphosphohydrolases (E-NTPDases) are a family of membrane-bound enzymes that hydrolyze extracellular di- and triphosphate nucleosides. E-NTPDases have been proposed to control extracellular nucleotide levels that mediate intercellular communication by binding to specific membrane receptors. Here we show a detailed immunocytochemical localization of two enzymes of the E-NTPDase family in the retinal layers of two vertebrate species, namely, the mouse and the zebrafish. In the mouse retina, NTPDase2 was chiefly localized in Müller glia and ganglion cell processes. NTPDase1 was located on neurons as well, since it was expressed by horizontal and ganglion cell processes, suggesting that nucleotides such as ATP and ADP can be hydrolyzed at the surface of these cells. NTPDase1 was also detected in intraretinal blood vessels of the mouse. Regarding zebrafish, NTPDases1 and 2 seem to be differentially localized in horizontal cell processes, photoreceptor segments, and ganglion cell dendrites and axons, but absent from Müller glia. Moreover, NTPDases1 and 2 appear to be expressed within the germinal margin of the zebrafish retina that contains proliferative and differentiating cells. Retinal homogenates from both species exhibited ecto-ATPase activity which might be attributed at least to NTPDases1 and 2, whose expression is described in this report. Our results suggest a compartmentalized regulation of extracellular nucleotide/nucleoside concentration in the retinal layers, supporting a relevant role for extracellular nucleotide mediated-signaling in vertebrate retinas.

Preservation of cochlear function in Cd39 deficient mice

Signalling actions of extracellular nucleotides via P2 receptors influence cellular function in most tissues. In the inner ear, P2 receptor signaling is involved in many processes including the regulation of hearing sensitivity and the cochlea's response to noise stress. CD39 (NTPDase1/ENTPD1) is an ectonucleotidase (ecto-nucleoside triphosphate diphosphohydrolase) that can hydrolyse purine and pyrimidine nucleoside tri- and di-phosphates to generate monophosphate nucleosides. Mice null for Cd39 exhibit major alterations in haemostasis and profound alterations in inflammatory and thrombotic reactions. Studies in the cochlea have suggested the involvement of purinergic-type signals that could be modulated by CD39 in regulation of cochlear blood flow and also auditory neurotransmission. This study aimed to determine the auditory phenotype of adult Cd39 null mice on the C57BL6 background. Auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE) were unaffected in Cd39-deficient mice across the range of test frequencies, suggesting normal neural and outer hair cell function. Mutant mice also showed little difference to wild type mice in vulnerability to acoustic trauma. Gene expression analysis of other membrane-bound NTPDases with comparable hydrolytic activity demonstrated an up-regulation of Entpd2 and Entpd8 in the cochleae of Cd39 deficient mice. These findings suggest that Cd39 deletion alone does not adversely affect cochlear function, possibly as compensatory up-regulation of other surface located NTPDases may offset predicted alterations in cochlear homeostasis.

Mechanical loading stimulates ecto-ATPase activity in human tendon cells

Response to external stimuli such as mechanical signals is critical for normal function of cells, especially when subjected to repetitive motion. Tenocytes receive mechanical stimuli from the load-bearing matrix as tension, compression, and shear stress during tendon gliding. Overloading a tendon by high strain, shear, or repetitive motion can cause matrix damage. Injury may induce cytokine expression, matrix metalloproteinase (MMP) expression and activation resulting in loss of biomechanical properties. These changes may result in tendinosis or tendinopathy. Alternatively, an immediate effector molecule may exist that acts in a signal-dampening pathway. Adenosine 5'-triphosphate (ATP) is a candidate signal blocker of mechanical stimuli. ATP suppresses load-inducible inflammatory genes in human tendon cells in vitro. ATP and other extracellular nucleotide signaling are regulated efficiently by two distinct mechanisms: purinoceptors via specific receptor-ligand binding and ecto-nucleotidases via the hydrolysis of specific nucleotide substrates. ATP is released from tendon cells by mechanical loading or by uridine 5'-triphosphate (UTP) stimulation. We hypothesized that mechanical loading might stimulate ecto-ATPase activity. Human tendon cells of surface epitenon (TSC) and internal compartment (TIF) were cyclically stretched (1 Hz, 0.035 strain, 2 h) with or without ATP. Aliquots of the supernatant fluids were collected at various time points, and ATP concentration (ATP) was determined by a luciferin-luciferase bioluminescence assay. Total RNA was isolated from TSC and TIF (three patients) and mRNA expression for ecto-nucleotidase was analyzed by RT-PCR. Human tendon cells secreted ATP in vitro (0.5-1 nM). Exogenous ATP was hydrolyzed within minutes. Mechanical load stimulated ATPase activity. ATP was hydrolyzed in mechanically loaded cultures at a significantly greater rate compared to no load controls. Tenocytes (TSC and TIF) expressed ecto-nucleotidase mRNA (ENTPD3 and ENPP1, ENPP2). These data suggest that motion may release ATP from tendon cells in vivo, where ecto-ATPase may also be activated to hydrolyze ATP quickly. Ecto-ATPase may act as a co-modulator in ATP load-signal modulation by regulating the half-life of extracellular purine nucleotides. The extracellular ATP/ATPase system may be important for tendon homeostasis by protecting tendon cells from responding to excessive load signals and activating injurious pathways.

Purinergic signaling in the inner ear

Epithelial cells of the inner ear coordinate their ion transport activity through a number of mechanisms. One important mechanism is the autocrine and paracrine signaling among neighboring cells in the ear via nucleotides, such as adenosine, ATP and UTP. This review summarizes observations on the release, detection and degradation of nucleotides by epithelial cells of the inner ear. Purinergic signaling is thought to be important for endolymph ion homeostasis and for protection from acoustic over-stimulation.

Activation-dependent trafficking of NTPDase2 in Chinese hamster ovary cells

Membrane-bound NTPDase2 is a member of the ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) enzyme family involved in the regulation of P2 receptor signaling. NTPDase2 has broad substrate specificity for extracellular nucleotides, but hydrolyses nucleoside 5'-triphosphates with high preference over nucleoside 5'-diphosphates. In this study, we have sought to determine how enzyme substrates acting on P2 receptors affect intracellular NTPDase2 trafficking. To achieve this, Chinese hamster ovary (CHO) cells were transiently transfected with rat-specific NTPDase2 cDNA tagged with green fluorescent protein (GFP), to allow direct visualisation of subcellular localisation and trafficking of NTPDase2. Cells were superfused with NTPDase2 substrates (ATP and UTP) and synthetic nucleotide analogues (ATPgammaS and ADPbetaS), and confocal image stacks were acquired at regular time intervals. NTPDase2 incorporation into the plasma membrane was determined by comparative analysis of fluorescence intensity in the cytosolic and membrane compartments. GFP-tagged NTPDase2 was fully functional and ATP and ATPgammaS induced membrane incorporation of GFP-NTPDase2 from putative intracellular stores, whilst UTP and ADPbetaS were ineffective. The increased ATP hydrolysis rate correlated with increased NTPDase2 trafficking to the plasma membrane. ATP-induced NTPDase2 trafficking was mediated by activation of endogenous P2X receptors involving Ca2+ entry rather than by P2Y receptor-induced release of Ca2+ from intracellular stores. Our results suggest that P2X receptor activation stimulates insertion of latent NTPDase2 into the plasma membrane. The increase in surface-located NTPDase2 may reflect a regulatory mechanism counteracting excessive stimulation and desensitisation of P2 receptors.

Nucleoside triphosphate diphosphohydrolase-2 is the ecto-ATPase of type I cells in taste buds

The presence of one or more calcium-dependent ecto-ATPases (enzymes that hydrolyze extracellular 5'-triphosphates) in mammalian taste buds was first shown histochemically. Recent studies have established that dominant ecto-ATPases consist of enzymes now called nucleoside triphosphate diphosphohydrolases (NTPDases). Massively parallel signature sequencing (MPSS) from murine taste epithelium provided molecular evidence suggesting that NTPDase2 is the most likely member present in mouse taste papillae. Immunocytochemical and enzyme histochemical staining verified the presence of NTPDase2 associated with plasma membranes in a large number of cells within all mouse taste buds. To determine which of the three taste cell types expresses this enzyme, double-label assays were performed with antisera directed against the glial glutamate/aspartate transporter (GLAST), the transduction pathway proteins phospholipase Cbeta2 (PLCbeta2) or the G-protein subunit alpha-gustducin, and serotonin (5HT) as markers of type I, II, and III taste cells, respectively. Analysis of the double-labeled sections indicates that NTPDase2 immunoreactivity is found on cell processes that often envelop other taste cells, reminiscent of type I cells. In agreement with this observation, NTPDase2 was located to the same membrane as GLAST, indicating that this enzyme is present in type I cells. The presence of ecto-ATPase in taste buds likely reflects the importance of ATP as an intercellular signaling molecule in this system.

The E-NTPDase family of ectonucleotidases: Structure function relationships and pathophysiological significance

Ectonucleotidases are ectoenzymes that hydrolyze extracellular nucleotides to the respective nucleosides. Within the past decade, ectonucleotidases belonging to several enzyme families have been discovered, cloned and characterized. In this article, we specifically address the cell surface-located members of the ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase/CD39) family (NTPDase1,2,3, and 8). The molecular identification of individual NTPDase subtypes, genetic engineering, mutational analyses, and the generation of subtype-specific antibodies have resulted in considerable insights into enzyme structure and function. These advances also allow definition of physiological and patho-physiological implications of NTPDases in a considerable variety of tissues. Biological actions of NTPDases are a consequence (at least in part) of the regulated phosphohydrolytic activity on extracellular nucleotides and consequent effects on P2-receptor signaling. It further appears that the spatial and temporal expression of NTPDases by various cell types within the vasculature, the nervous tissues and other tissues impacts on several patho-physiological processes. Examples include acute effects on cellular metabolism, adhesion, activation and migration with other protracted impacts upon developmental responses, inclusive of cellular proliferation, differentiation and apoptosis, as seen with atherosclerosis, degenerative neurological diseases and immune rejection of transplanted organs and cells. Future clinical applications are expected to involve the development of new therapeutic strategies for transplantation and various inflammatory cardiovascular, gastrointestinal and neurological diseases.

Asphyxia and depolarization increase adenosine levels in perilymph

Extracellular adenosine has been suggested as a modulator of cochlear function. To date the release of adenosine into the extracellular spaces of the cochlea has not been demonstrated. Therefore, experiments were designed to examine whether adenosine release into perilymph could be detected in response to depolarization by high potassium concentrations or in response to asphyxia. For this purpose, the perilymph compartment of guinea pigs was perfused with an artificial perilymph and the effluent assayed for ATP, ADP, AMP and adenosine. Results indicate that potassium induced a slight, significant increase and asphyxia induced a very large, significant increase in adenosine levels in perilymph effluent. No changes in the levels of the other compounds were measured. It is concluded that depolarization and asphyxia can induce the release of adenosine into perilymph.

Immunolocalization of ecto-nucleoside triphosphate diphosphohydrolase 3 in rat brain: Implications for modulation of multiple homeostatic systems including feeding and sleep–wake behaviors

Three anti-peptide antisera were raised against three distinct amino acid sequences of ecto-nucleoside triphosphate diphosphohydrolase 3 (NTPDase3), characterized by Western blot analyses, and used to determine the distribution of NTPDase3 protein in adult rat brain. The three antisera all yielded similar immunolocalization data, leading to increased reliability of the results obtained. Unlike NTPDase1 and NTPDase2, NTPDase3 immunoreactivity was detected exclusively in neurons. Immunoreactivity was localized primarily to axon-like structures with prominent staining of presynaptic elements. Specific perikaryal immunostaining was detected primarily in scattered neurons near the lateral hypothalamic area and the perifornical nucleus. High densities of immunoreactive axon-like fibers were present in midline regions of the forebrain and midbrain. Highly scattered NTPDase3 positive fibers were observed in the cerebral cortex, the hippocampal formation, and the basal ganglia. Moreover, very high densities of immunostained fibers were detected in the mediobasal hypothalamus, with the overall mesencephalic pattern of staining associated closely with hormone responsive nuclei. High densities of NTPDase3 positive terminals were also associated with noradrenergic neurons. However, co-immunolocalization studies revealed clearly that NTPDase3 immunoreactivity was not localized within the noradrenaline cells or terminals. In contrast, nearly all of the NTPDase3 immunopositive hypothalamic cells, and most fibers in the mid- and hindbrain, also expressed hypocretin-1/orexin-A. The overall pattern of expression and co-localization with hypocretin-1/orexin-A suggests that NTPDase3, by regulating the extracellular turnover of ATP, may modulate feeding, sleep-wake, and other behaviors through diverse homeostatic systems.

C-terminal splicing of NTPDase2 provides distinctive catalytic properties, cellular distribution and enzyme regulation

The present study provides functional characterization of alternative splicing of the NTPDase2 (ecto-nucleoside triphosphate diphosphohydrolase-2) involved in the regulation of extracellular nucleotide concentrations in a range of organ systems. A novel NTPDase2beta isoform produced by alternative splicing of the rat NTPDase2 gene provides an extended intracellular C-terminus and distinguishes itself from NTPDase2alpha isoform in gaining several intracellular protein kinase CK2 (casein kinase 2) phosphorylation sites and losing the intracellular protein kinase C motif. The plasmids containing NTPDase2alpha or NTPDase2beta cDNA were used to stably transfect Chinese-hamster ovary-S cells. Imaging studies showed that NTPDase2alpha was predominantly membrane-bound, whereas NTPDase2beta had combined cell surface and intracellular localization. alpha and beta isoforms showed variations in divalent cation dependence and substrate specificity for nucleoside-5'-triphosphates and nucleoside-5'-diphosphates. NTPDase2beta exhibited reduced ATPase activity and no apparent ADPase activity. NTPDase2 isoforms demonstrated similar sensitivity to inhibitors such as suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid, and differential regulation by protein kinases. NTPDase2beta was up-regulated by intracellular protein kinase CK2 phosphorylation, whereas NTPDase2alpha activity was down-regulated by protein kinase C phosphorylation. The results demonstrate that alternative coding of the intracellular C-terminal domain contributes distinctive phenotypic variation with respect to extracellular nucleotide specificity, hydrolysis kinetics, protein kinase-dependent intracellular regulation and protein trafficking. These findings advance the molecular physiology of this enzyme system by characterizing the contribution of the C-terminal domain to many of the enzyme's signature properties.

Expression of NTPDase1 and NTPDase2 in murine kidney: relevance to regulation of P2 receptor signaling

The regulation of renal function by extracellular nucleotides encompasses alterations in glomerular hemodynamics, microvascular function, tubuloglomerular feedback, tubular transport, cell growth or apoptosis, and transport of water and solutes in the medullary collecting duct. Nearly all cells can release ATP or other nucleotides that are then rapidly hydrolyzed in the extracellular milieu. However, little information is available on the cellular expression of ectoenzymes that hydrolyze extracellular nucleotides within the kidney. Nucleoside triphosphate diphosphohydrolases (NTPDases) are plasma membrane-bound ectonucleotidases. NTPDase1 has identity with CD39, a B lymphocyte activation marker, and hydrolyzes extracellular ATP and ADP to AMP within the vasculature, whereas NTPDase2/CD39L(ike)1 preferentially converts ATP to ADP outside of blood vessels. Using immunohistochemical and in situ hybridization approaches, we localized the protein and mRNA of NTPDase1 and 2 in murine renal tissues. In the renal cortex, NTPDase1 is expressed by vascular smooth muscle cells and endothelium in interlobular arteries, afferent glomerular arterioles, and peritubular capillaries. In the inner medulla, NTPDase1 is expressed in ascending thin limbs of Henle's loop, ducts of Bellini, and in the pelvic wall. In contrast, NTPDase2 is expressed in Bowman's capsule, glomerular arterioles, adventitia of blood vessels, and pelvic wall. Thus the distribution patterns of NTPDases have parallels to the known distribution of P2 receptors within the kidney. NTPDases may modulate regulatory effects of ATP and degradation products within the vasculature and other sites and thereby potentially influence physiological as well as multiple pathological events in the kidney.

Regulation of P2Y1 receptor-mediated signaling by the ectonucleoside triphosphate diphosphohydrolase isozymes NTPDase1 and NTPDase2

Ectonucleoside triphosphate diphosphohydrolases (NTPDases) control the concentration of released extracellular nucleotides, but the precise physiological roles played by these isozymes in modulation of P2 receptor signaling remain unclear. Activation of the human P2Y(1) receptor was studied in the presence of NTPDase1 or NTPDase2 expressed either in the same cell as the receptor or in P2Y(1) receptor-expressing cells cocultured with NTPDaseexpressing cells. Coexpression of NTPDase1 with the P2Y(1) receptor resulted in increases in the EC(50) for 2'-methylthioadenosine 5'-diphosphate (2MeSADP; 12-fold), ADP (50-fold), and ATP (10-fold) for activation of phospholipase C. Similar effects were observed when the P2Y(1) receptor and NTPDase1 were expressed on different cells. These results are explained by the capacity of NTPDase1 to hydrolyze both nucleoside triphosphates and diphosphates. NTPDase2 preferentially hydrolyzes nucleoside triphosphates, and the presence of NTPDase2 under either coexpression or coculture conditions did not change the EC(50) of 2MeSADP, ADP, or adenosine 5'-O-(2-thiodiphosphate) for activation of the P2Y(1) receptor. However, the EC(50) for ATP was 15-fold lower in the presence of NTPDase2 than in cells expressing the P2Y(1) receptor alone. Whereas expression of NTPDase1 decreased basal activity of the P2Y(1) receptor, the presence of the NTPDase2 resulted in P2Y(1) receptor-dependent increases in basal activity. These results suggest that basal activity of the P2Y(1) receptor is maintained by paracrine or autocrine release of receptor agonists and that the biological and/or pharmacological response mediated by P2Y receptors in target tissues is highly dependent on the types of ectonucleotidases expressed in the vicinity of the receptor.

Noise exposure induces up-regulation of ecto-nucleoside triphosphate diphosphohydrolases 1 and 2 in rat cochlea

Extracellular ATP acting via P2 receptors in the inner ear initiates a variety of signaling pathways that may be involved in noise-induced cochlear injury. Nucleoside triphosphate diphosphohydrolase (NTPDase)1/CD39 and NTPDase2/CD39L1 are key elements for regulation of extracellular nucleotide concentrations and P2 receptor signaling in the cochlea. This study characterized the effect of noise exposure on regulation of NTPDase1 and NTPDase2 expression in the cochlea using a combination of real-time RT-PCR, immunohistochemistry and functional studies. Adult Wistar rats were exposed to broad band noise at 90 dB and 110 dB sound pressure level (SPL) for 72 h. Exposure to 90 dB SPL induced a small and temporary change of auditory thresholds (temporary threshold shift), while exposure to 110 dB SPL induced a robust and permanent change of auditory thresholds (permanent threshold shift). NTPDase1 and NTPDase2 mRNA transcripts were upregulated in the cochlea exposed to 110 dB SPL, while mild noise (90 dB SPL) altered only NTPDase1 mRNA expression levels. Changes in NTPDases expression did not correlate with levels of circulating corticosterone, implying that the up-regulation of NTPDases expression was not stress-related. Semi-quantitative immunohistochemistry in the cochlea exposed to 110 dB SPL localized the increased NTPDase1 and NTPDase2 immunostaining in the stria vascularis and up-regulation of NTPDase2 in the intraganglionic spiral bundle. In contrast, NTPDase1 was down-regulated in the cell bodies of the spiral ganglion neurones. Distribution of NTPDases was not altered in the cochlea exposed to 90 dB SPL. Functional studies revealed increased ectonucleotidase activities in the cochlea after exposure to 110 dB SPL, consistent with up-regulation of NTPDases. The changes in NTPDases expression may reflect adaptive response of cochlear tissues to limit ATP signaling during noise exposure.

Localization of nucleoside triphosphate diphosphohydrolase-1 (NTPDase1) and NTPDase2 in pancreas and salivary gland

Ectonucleoside triphosphate diphosphohydrolases (NTPDases) are membrane-bound ectoenzymes that hydrolyze extracellular nucleotides. We investigated the distribution of NTPDase1 and NTPDase2 in murine salivary gland and pancreas. Histochemistry and immunostaining (by both light and electron microscopy), combined with functional assays, were used to describe the localization patterns and enzyme activities in the organs of wild-type and NTPDase1/cd39-null mice. Pancreatic acinar cells and salivary gland acinar/myoepithelial cells were positive for NTPDase1 and NTPDase2. Ecto-ATPase activity was slightly higher in salivary glands. Ductal epithelial cells expressed ecto-ATPase activity but NTPDase1 and NTPDase2 expression were weak at best. ATPase activity was found in blood vessels of both tissues and its localization pattern overlapped with NTPDase1 staining. In these structures, NTPDase2 antibodies stained the basolateral aspect of endothelial cells and the supporting cells. Biochemical assays and histochemical staining showed relatively high levels of ATPase activity in both glands of cd39(-/-) mice. Our data therefore support a physiological role for NTPDase2 and other ectonucleotidases in the pancreas and salivary glands. Because NTPDase1 is expressed in non-vascular cell types, this finding suggests that NTPDase1 may have functions in the gastrointestinal tract that differ from those demonstrated in the vascular system.

Association of the ecto-ATPase NTPDase2 with glial cells of the peripheral nervous system

Cellular signaling via extracellular nucleotides appears to play a major role in the functioning of the peripheral nervous system. Information regarding the functional characterization of nucleotide P2 receptors or their expression pattern has been accumulating rapidly; however, very little is known regarding the distribution of ecto-nucleotidases in the periphery. The extracellular level of nucleotides is controlled by ecto-nucleotidases, whereby the three membrane-bound members of the ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) family are of special functional importance. Using enzyme histochemistry and immunostaining, we demonstrate that NTPDase2 is associated with nonmyelinating Schwann cells of the rat sciatic nerve, whereas NTPDase1 is restricted to blood vessel walls. NTPDase2 immunoreactivity was detected from embryonic day E18 onward, suggesting that immature Schwann cells express the enzyme. With the onset of myelination, NTPDase2 immunoreactivity remained associated solely with nonmyelinating Schwann cells. NTPDase2 was absent from perisynaptic Schwann cells but was associated with fibroblasts covering the endplate at some distance. In addition, NTPDase2 immunoreactivity was associated with the satellite glial cells in dorsal root ganglia and sympathetic ganglia, and with the enteric glia surrounding the cell bodies of ganglionic neurons of the myenteric and the submucous plexus. In contrast to NTPDase1, NTPDase2 preferentially hydrolyzes nucleoside triphosphates over nucleoside diphosphates and thus can act either in inactivating or in producing P2 receptor ligands. Our results suggest that NTPDase2 plays an important role in the control of nucleotide-mediated activation of peripheral neurons or glia and in the dialogue between these two cell types.