Na,K-ATPase beta subunit isoform expression in the peripheral nervous system of the rat

Terpenes in Cannabis sativa Inhibit Capsaicin Responses in Rat DRG Neurons via Na+/K+ ATPase Activation

Terpenes in Cannabis sativa exert analgesic effects, but the mechanisms are uncertain. We examined the effects of 10 terpenes on capsaicin responses in an established model of neuronal hypersensitivity. Adult rat DRG neurons cultured with neurotrophic factors NGF and GDNF were loaded with Fura2AM for calcium imaging, and treated with individual terpenes or vehicle for 5 min, followed by 1 µMol capsaicin. In vehicle treated control experiments, capsaicin elicited immediate and sustained calcium influx. Most neurons treated with terpenes responded to capsaicin after 6-8 min. Few neurons showed immediate capsaicin responses that were transient or normal. The delayed responses were found to be due to calcium released from the endoplasmic reticulum, as they were maintained in calcium/magnesium free media, but not after thapsigargin pre-treatment. Terpene inhibition of calcium influx was reversed after washout of medium, in the absence of terpenes, and in the presence of the Na+/K+ ATPase inhibitor ouabain, but not CB1 or CB2 receptor antagonists. Thus, terpenes inhibit capsaicin evoked calcium influx by Na+/K+ ATPase activation. Immunofluorescence showed TRPV1 co-expression with α1β1 Na+/K+ ATPase in most neurons while others were either TRPV1 or α1β1 Na+/K+ ATPase positive.

Expression of Na/K-ATPase subunits in the human cochlea: a confocal and super-resolution microscopy study with special reference to auditory nerve excitation and cochlear implantation

Background: For the first time the expression of the ion transport protein sodium/potassium-ATPase and its isoforms was analyzed in the human cochlea using light- and confocal microscopy as well as super-resolution structured illumination microscopy. It may increase our understanding of its role in the propagation and processing of action potentials in the human auditory nerve and how electric nerve responses are elicited from auditory prostheses. Material and methods: Archival human cochlear sections were obtained from trans-cochlear surgeries. Antibodies against the Na/K-ATPase β1 isoform together with α1 and α3 were used for immunohistochemistry. An algorithm was applied to assess the expression in various domains. Results: Na/K ATPase β1 subunit was expressed, mostly combined with the α1 isoform. Neurons expressed the β1 subunit combined with α3, while satellite glial cells expressed the α1 isoform without recognized association with β1. Types I and II spiral ganglion neurons and efferent fibers expressed the Na/K-ATPase α3 subunit. Inner hair cells, nerve fibers underneath, and efferent and afferent fibers in the organ of Corti also expressed α1. The highest activity of Na/K-ATPase β1 was at the inner hair cell/nerve junction and spiral prominence. Conclusion: The human auditory nerve displays distinct morphologic features represented in its molecular expression. It was found that electric signals generated via hair cells may not go uninterrupted across the spiral ganglion, but are locally processed. This may be related to particular filtering properties in the human acoustic pathway.

Presynaptic inhibition of nociceptive neurotransmission by somatosensory neuron-secreted suppressors

Noxious stimuli cause pain by activating cutaneous nociceptors. The Aδ- and C-fibers of dorsal root ganglion (DRG) neurons convey the nociceptive signals to the laminae I-II of spinal cord. In the dorsal horn of spinal cord, the excitatory afferent synaptic transmission is regulated by the inhibitory neurotransmitter γ-aminobutyric acid and modulators such as opioid peptides released from the spinal interneurons, and by serotonin, norepinepherine and dopamine from the descending inhibitory system. In contrast to the accumulated evidence for these central inhibitors and their neural circuits in the dorsal spinal cord, the knowledge about the endogenous suppressive mechanisms in nociceptive DRG neurons remains very limited. In this review, we summarize our recent findings of the presynaptic suppressive mechanisms in nociceptive neurons, the BNP/NPR-A/PKG/BKCa channel pathway, the FSTL1/α1Na⁺-K⁺ ATPase pathway and the activin C/ERK pathway. These endogenous suppressive systems in the mechanoheat nociceptors may also contribute differentially to the mechanisms of nerve injury-induced neuropathic pain or inflammation-induced pain.

FXYD2, a γ subunit of Na⁺, K⁺-ATPase, maintains persistent mechanical allodynia induced by inflammation

Na⁺, K⁺-ATPase (NKA) is required to generate the resting membrane potential in neurons. Nociceptive afferent neurons express not only the α and β subunits of NKA but also the γ subunit FXYD2. However, the neural function of FXYD2 is unknown. The present study shows that FXYD2 in nociceptive neurons is necessary for maintaining the mechanical allodynia induced by peripheral inflammation. FXYD2 interacted with α1NKA and negatively regulated the NKA activity, depolarizing the membrane potential of nociceptive neurons. Mechanical allodynia initiated in FXYD2-deficient mice was abolished 4 days after inflammation, whereas it persisted for at least 3 weeks in wild-type mice. Importantly, the FXYD2/α1NKA interaction gradually increased after inflammation and peaked on day 4 post inflammation, resulting in reduction of NKA activity, depolarization of neuron membrane and facilitation of excitatory afferent neurotransmission. Thus, the increased FXYD2 activity may be a fundamental mechanism underlying the persistent hypersensitivity to pain induced by inflammation.

Regulation and pharmacological blockade of sodium-potassium ATPase: a novel pathway to neuropathy

Inflammation causes upregulation of NaV1.7 sodium channels in the associated dorsal root ganglia (DRG). The resultant increase in sodium influx must be countered to maintain osmotic homeostasis. The primary mechanism to pump sodium out of neurons is Na(+), K(+)-ATPase. To test whether there is a compensatory upregulation of Na(+), K(+)-ATPase after inflammation, rats received an injection of complete Freund's adjuvant (CFA) into one hindpaw and saline into the contralateral hindpaw. Three days later, L4-L6 DRGs were extracted and analyzed using gel electrophoresis and immunohistochemistry. Immunoreactivity for both the α-1 and α-3 subunits were increased in DRG associated with CFA-treatment, compared to saline-treatment. To test whether dysregulation of Na(+), K(+)-ATPase may cause cell death after inflammation, we produced a pharmacological blockade with ouabain (10mg/kg, s.c.) three days after CFA injection and paws were stimulated or not. Twenty-four hours later, DRG were removed and stained with cresyl violet. Greater cell death was seen in DRG from ouabain-treated animals on the CFA treated side than the saline-treated side. Paw stimulation doubled this difference. Control DRG showed little neuronal death. These results are evidence that regulation of Na(+), K(+)-ATPase during major inflammatory disease states is critical for homeostatic protection of primary afferent neurons.

Follistatin-like 1 suppresses sensory afferent transmission by activating Na+,K+-ATPase

Excitatory synaptic transmission is modulated by inhibitory neurotransmitters and neuromodulators. We found that the synaptic transmission of somatic sensory afferents can be rapidly regulated by a presynaptically secreted protein, follistatin-like 1 (FSTL1), which serves as a direct activator of Na(+),K(+)-ATPase (NKA). The FSTL1 protein is highly expressed in small-diameter neurons of the dorsal root ganglion (DRG). It is transported to axon terminals via small translucent vesicles and secreted in both spontaneous and depolarization-induced manners. Biochemical assays showed that FSTL1 binds to the α1 subunit of NKA and elevates NKA activity. Extracellular FSTL1 induced membrane hyperpolarization in cultured cells and inhibited afferent synaptic transmission in spinal cord slices by activating NKA. Genetic deletion of FSTL1 in small DRG neurons of mice resulted in enhanced afferent synaptic transmission and sensory hypersensitivity, which could be reduced by intrathecally applied FSTL1 protein. Thus, FSTL1-dependent activation of NKA regulates the threshold of somatic sensation.

Localization and irregular distribution of Na,K-ATPase in myelin sheath from rat sciatic nerve

Sodium, potassium adenosine triphosphatase (Na,K-ATPase) is a membrane-bound enzyme that maintains the Na(+) and K(+) gradients used in the nervous system for generation and transmission of bioelectricity. Recently, its activity has also been demonstrated during nerve regeneration. The present study was undertaken to investigate the ultrastructural localization and distribution of Na,K-ATPase in peripheral nerve fibers. Small blocks of the sciatic nerves of male Wistar rats weighing 250-300g were excised, divided into two groups, and incubated with and without substrate, the para-nitrophenyl phosphate (pNPP). The material was processed for transmission electron microscopy, and the ultra-thin sections were examined in a Philips CM 100 electron microscope. The deposits of reaction product were localized mainly on the axolemma, on axoplasmic profiles, and irregularly dispersed on the myelin sheath, but not in the unmyelinated axons. In the axonal membrane, the precipitates were regularly distributed on the cytoplasmic side. These results together with published data warrant further studies for the diagnosis and treatment of neuropathies with compromised Na,K-ATPase activity.

Na,K-ATPase Subunit Heterogeneity as a Mechanism for Tissue-Specific Ion Regulation

The Na,K-ATPase comprises a family of isozymes that catalyze the active transport of cytoplasmic Na+ for extracellular K+ at the plasma membrane of cells. Isozyme diversity for the Na,K-ATPase results from the association of different molecular forms of the alpha (alpha1, alpha2, alpha3, and alpha4) and beta (beta1, beta2, and beta3) subunits that constitute the enzyme. The various isozymes are characterized by unique enzymatic properties and a highly regulated pattern of expression that depends on cell type, developmental stage, and hormonal stimulation. The molecular complexity of the Na,K-ATPase goes beyond its alpha and beta isoforms and, in certain tissues, other accessory proteins associate with the enzyme. These small membrane-bound polypeptides, known as the FXYD proteins, modulate the kinetic characteristics of the Na,K-ATPase. The experimental evidence available suggests that the molecular and functional heterogeneity of the Na,K-ATPase is a physiologically relevant event that serves the specialized functions of cells. This article focuses on the functional properties, regulation, and the biological relevance of the Na,K-ATPase isozymes as a mechanism for the tissue-specific control of Na+ and K+ homeostasis.

Properties of the Na+/K+ pump current in small neurons from adult rat dorsal root ganglia

1 The present investigation was undertaken to characterize the Na(+)/K(+) pump current in small ( Collapse

Key Words

• na⁺/k⁺ pump current
• dorsal root ganglion
• neuron
• ouabain
• α1 isoform
• α3 isoform

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MESH Headings

• Animals
• Cell Size/drug effects
• Cell Size/physiology
• Cells, Cultured
• Dose-Response Relationship, Drug
• Ganglia, Spinal/cytology
• Ganglia, Spinal/drug effects
• Ganglia, Spinal/physiology
• Male
• Membrane Potentials/drug effects
• Membrane Potentials/physiology
• Neurons/cytology
• Neurons/drug effects
• Neurons/physiology
• Ouabain/pharmacology
• Rats
• Rats, Sprague-Dawley
• Sodium-Potassium-Exchanging ATPase/physiology

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Affiliation(s)

Kanako Hamada Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Hiroshi Matsuura Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Mitsuru Sanada Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Futoshi Toyoda Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Mariko Omatsu-Kanbe Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Atsunori Kashiwagi Division of Endocrinology and Metabolism, Department of Medicine, Otsu, Shiga University of Medical Science, Shiga 520-2192, Japan
Hitoshi Yasuda Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan

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Sodium- and potassium-activated ATPase in the mammalian vestibular system

Autoradiographic and cytochemical procedures were employed to determine the cellular distribution of the Na,K-ATPase enzyme in the mammalian vestibular system. A light-microscope survey of vestibular tissues incubated with [(3)H]ouabain shows high densities of ouabain binding sites within the dark cell epithelium (DC) of the ampullae of the semi-circular canals, and to a lesser extent, the DC of the utricular macula. A moderate number of binding sites was found in nerve fibers penetrating the connective tissue beneath the sensory epithelium (SE) of the ampullae and the maculae. A small number of binding sites is distributed in the deep portion of the SE, both in the ampullae and in the maculae. These latter binding sites seem to be associated with nerve terminals and receptor cells. At the ultrastructural level, the vestibular dark cells exhibit extensive basolateral membrane infolding, a morphological hallmark of cells engaged in trans-epithelial ion transport. The cytochemical reaction product is K(+)-dependent, ouabain inhibitable, and is restricted to the basolateral membrane extensions, with little or no product on the luminal membrane. The extent of membrane infolding in dark cells of the utricle is less pronounced than that of the ampullar dark cells and the intensity of the cytochemical reaction appears to correlate with the extent of membrane infolding. The results support the widely held hypothesis that the vestibular dark cells play a role in endolymph production. They also suggest that the vestibular sensory epithelia may be a site of ion exchange.

Functional Na+/K+ pump in rat dorsal root ganglia neurons

Steady-state Na+/K+ pump current (Ip) in isolated adult rat dorsal root ganglia neurons was studied to determine if the plasma membrane Na+/K+ pump activity is uniform across the population of dorsal root ganglia neurons. Cells were voltage-clamped at -40 mV and holding current (Ih) was recorded using whole-cell patch-clamp techniques under conditions that stimulate the Na+/K+ pump (60 mM intracellular Na+ and 5.4 mM extracellular K+). Ip was defined as the 1 mM ouabain-sensitive fraction of Ih. Data suggest the existence of three subpopulations of dorsal root ganglia neurons having mean steady-state Ip densities of 1.6+/-0.1, 3.8+/-0.2 and 7.5+/-0.4 pA/pF. Neurons with small Ip had an average soma perimeter of 95+/-3 microm, while neurons with medium and large Ip density had significantly larger soma sizes (131+/-8 and 141+/-7 microm, respectively). Neurons with a large Ip density had a significantly lower specific membrane resistance (Rm; mean 4.0+/-0.3 kohm x cm2) than neurons with medium or small Ip density (19+/-6 and 31+/-6 kohm x cm2, respectively). Regardless of these differences, in all groups of neurons Ip had a low sensitivity to ouabain (Ip half inhibition by ouabain was observed at 80-110 microM). These data suggest that the Na+/K+ pump site density and/or its activity is not uniform throughout the dorsal root ganglia neuron population; however, this non-uniformity does not appear to relate to the functional expression of the different alpha isoforms of the Na+/K+ pump. The major functional Na+/K+ pump in the dorsal root ganglia neuron plasma membrane appeared to be the low ouabain affinity (alpha1) isoform.

Regulation of the Na+/K+-ATPase by insulin: why and how?

The sodium-potassium ATPase (Na+/K+-ATPase or Na+/K+-pump) is an enzyme present at the surface of all eukaryotic cells, which actively extrudes Na+ from cells in exchange for K+ at a ratio of 3:2, respectively. Its activity also provides the driving force for secondary active transport of solutes such as amino acids, phosphate, vitamins and, in epithelial cells, glucose. The enzyme consists of two subunits (alpha and beta) each expressed in several isoforms. Many hormones regulate Na+/K+-ATPase activity and in this review we will focus on the effects of insulin. The possible mechanisms whereby insulin controls Na+/K+-ATPase activity are discussed. These are tissue- and isoform-specific, and include reversible covalent modification of catalytic subunits, activation by a rise in intracellular Na+ concentration, altered Na+ sensitivity and changes in subunit gene or protein expression. Given the recent escalation in knowledge of insulin-stimulated signal transduction systems, it is pertinent to ask which intracellular signalling pathways are utilized by insulin in controlling Na+/K+-ATPase activity. Evidence for and against a role for the phosphatidylinositol-3-kinase and mitogen activated protein kinase arms of the insulin-stimulated intracellular signalling networks is suggested. Finally, the clinical relevance of Na+/K+-ATPase control by insulin in diabetes and related disorders is addressed.

Plasticity of Na,K-ATPase isoform expression in cultures of flat astrocytes: species differences in gene expression

The Na,K-ATPase plays an active role in glial physiology, contributing to K+ uptake as well as to the Na+ gradients used by other membrane carriers. There are multiple isoforms of Na,K-ATPase alpha and beta subunits, and different combinations result in different affinities for Na+ and K+. Isoform choice should thus influence K+ and Na+ homeostasis in astrocytes. Prior studies of astrocyte Na,K-ATPase subunit composition have produced apparently conflicting results, suggesting plasticity of gene expression. Purified flat astrocytes from the cerebral cortex and cerebellum of both mouse and rat were systematically investigated here. Using antibodies specific for the alpha1, alpha2, alpha3, beta1, beta2, and beta3 subunits, isoform level was assessed with Western blots, and cellular distribution was visualized with immunofluorescence. Although alpha1 was always expressed, differences were observed in the expression of alpha2 and beta2, subunits that can be expressed in astrocytes in vivo and in coculture with neurons. In addition, abundant alpha subunit was expressed in rat astrocytes and in mouse cerebellar astrocytes without an equivalent level of any of the known beta isoforms, suggesting that an additional beta subunit important for glia is yet to be discovered. Conditions that have been shown to increase Na,K-ATPase activity in astrocyte cultures, such as dibutyryl cAMP, high extracellular K+, and glutamate, did not specifically induce missing subunits, suggesting that cellular interactions are required to alter the ion transporter phenotype.

Immunocytochemical demonstration of beta 1-subunit of Na+/K(+)-ATPase in the mechanoreceptive Ruffini-like endings of the rat incisor ligament

The localization of one of the isoforms of Na+/K(+)-ATPase, the beta 1-subunit, was investigated in the periodontal Ruffini endings of rat incisors by light- and electron-microscopic immunocytochemistry. Immunoreactivity for the rat beta 1-subunit followed the pattern of dendritic terminal arborization in the alveolar half of the lingual periodontal ligament. Ultrastructurally, the reaction products were localized in dilatations of axons, possibly the terminals of Ruffini-like endings in the periodontal ligament. No immunoreactivity was seen in Schwann cells. The immunostaining results support the view that the beta 1-subunit of Na+/K(+)-ATPase is the predominant isoform in sensory neurones, and that this protein is a useful marker for periodontal Ruffini-like endings.