Differential expression of Na,K-ATPase ?-isoform mRNAs in aging rat cerebellum

Comparative description of the mRNA expression profile of Na+ /K+ -ATPase isoforms in adult mouse nervous system

Mutations in genes encoding Na⁺ /K⁺ -ATPase α1, α2, and α3 subunits cause a wide range of disabling neurological disorders, and dysfunction of Na⁺ /K⁺ -ATPase may contribute to neuronal injury in stroke and dementia. To better understand the pathogenesis of these diseases, it is important to determine the expression patterns of the different Na⁺ /K⁺ -ATPase subunits within the brain and among specific cell types. Using two available scRNA-Seq databases from the adult mouse nervous system, we examined the mRNA expression patterns of the different isoforms of the Na⁺ /K⁺ -ATPase α, β and Fxyd subunits at the single-cell level among brain regions and various neuronal populations. We subsequently identified specific types of neurons enriched with transcripts for α1 and α3 isoforms and elaborated how α3-expressing neuronal populations govern cerebellar neuronal circuits. We further analyzed the co-expression network for α1 and α3 isoforms, highlighting the genes that positively correlated with α1 and α3 expression. The top 10 genes for α1 were Chn2, Hpcal1, Nrgn, Neurod1, Selm, Kcnc1, Snrk, Snap25, Ckb and Ccndbp1 and for α3 were Sorcs3, Eml5, Neurod2, Ckb, Tbc1d4, Ptprz1, Pvrl1, Kirrel3, Pvalb, and Asic2.

Region- and neuronal-subtype-specific expression of Na,K-ATPase alpha and beta subunit isoforms in the mouse brain

Na,K‐ATPase is a ubiquitous molecule contributing to the asymmetrical distribution of Na⁺ and K⁺ ions across the plasma membrane and maintenance of the membrane potential, a prerequisite of neuronal activity. Na,K‐ATPase comprises three subunits (α, β, and FXYD). The α subunit has four isoforms in mice, with three of them (α1, α2, and α3) expressed in the brain. However, the functional and biological significances of the different brain isoforms remain to be fully elucidated. Recent studies have revealed the association of Atp1a3, a gene encoding α3 subunit, with neurological disorders. To map the cellular distributions of the α subunit isoforms and their coexpression patterns, we evaluated the mRNA expression of Atp1a1, Atp1a2, and Atp1a3 by in situ hybridization in the mouse brain. Atp1a1 and Atp1a3 were expressed in neurons, whereas Atp1a2 was almost exclusively expressed in glial cells. Most neurons coexpressed Atp1a1 and Atp1a3, with highly heterogeneous expression levels across the brain regions and neuronal subtypes. We identified parvalbumin (PV)‐expressing GABAergic neurons in the hippocampus, somatosensory cortex, and retrosplenial cortex as an example of a neuronal subtype expressing low Atp1a1 and high Atp1a3. The expression of Atp1b isoforms was also heterogeneous across brain regions and cellular subtypes. The PV‐expressing neurons expressed a high level of Atp1b1 and a low level of Atp1b2 and Atp1b3. These findings provide basic information on the region‐ and neuronal‐subtype‐dependent expression of Na,K‐ATPase α and β subunit isoforms, as well as a rationale for the selective involvement of neurons expressing high levels of Atp1a3 in neurological disorders.

Differential expression patterns of sodium potassium ATPase alpha and beta subunit isoforms in mouse brain during postnatal development

The sodium potassium ATPase (Na⁺/K⁺ ATPase) is essential for the maintenance of a low intracellular Na⁺ and a high intracellular K⁺ concentration. Loss of function of the Na⁺/K⁺ ATPase due to mutations in Na⁺/K⁺ ATPase genes, anoxic conditions, depletion of ATP or inhibition of the Na⁺/K⁺ ATPase function using cardiac glycosides such as digitalis, causes a depolarization of the resting membrane potential. While in non-excitable cells, the uptake of glucose and amino acids is decreased if the function of the Na⁺/K⁺ ATPase is compromised, in excitable cells the symptoms range from local hyper-excitability to inactivating depolarization. Although several studies have demonstrated the differential expression of the various Na⁺/K⁺ ATPase alpha and beta isoforms in the brain tissue of rodents, their expression profile during development has yet to be thoroughly investigated. An immunohistochemical analysis of postnatal day 19 mouse brain showed ubiquitous expression of Na⁺/K⁺ ATPase isoforms α1, β1 and β2 in both neurons and glial cells, whereas α2 was expressed mostly in glial cells and the α3 and β3 isoforms were expressed in neurons. Furthermore, we examined potential changes in the relative expression of the different Na⁺/K⁺ ATPase isoforms in different brain areas of postnatal day 6 and in adult 9 months old animals using immunoblot analysis. Our results show a significant up-regulation of the α1 isoform in cortex, hippocampus and cerebellum, whereas, the α2 isoform was significantly up-regulated in midbrain. The β3 isoform showed a significant up-regulation in all brain areas investigated. The up-regulation of the α3 isoform matched that of the β2 isoform which were both significantly up-regulated in cortex, hippocampus and midbrain, suggesting that the increased maturation of the neuronal network is accompanied by an increase in expression of α3/β2 complexes in these brain structures.

A Transgenic Mouse Model to Selectively Identify α3 Na,K-ATPase Expressing Cells in the Nervous System

The α₃ Na⁺,K⁺-ATPase (α₃NKA) is one of four known α isoforms of the mammalian transporter. A deficiency in α₃NKA is linked to severe movement control disorders. Understanding the pathogenesis of these disorders is limited by an incomplete knowledge of α₃NKA expression in the brain as well as the challenges associated with identifying living cells that express the isoform for subsequent electrophysiological studies. To address this problem, transgenic mice were generated on the C57BL/6 genetic background, which utilize the mouse α₃ subunit gene (Atp1a3) promoter to drive the expression of ZsGreen1 fluorescent protein. Consistent with published results on α₃NKA distribution, a ZsGreen1 signal was detected in the brain, but not in the liver, with Atp1a3-ZsGreen1 transgenic mice. The intensity of ZsGreen1 fluorescence in neuronal cell bodies varied considerably in the brain, being highest in the brainstem, deep cerebellar and select thalamic nuclei, and relatively weak in cortical regions. Fluorescence was not detected in astrocytes or white matter areas. ZsGreen1-positive neurons were readily observed in fresh (unfixed) brain sections, which were amenable to patch-clamp recordings. Thus, the α₃NKA-ZsGreen1 mouse model provides a powerful tool for studying the distribution and functional properties of α₃NKA-expressing neurons in the brain.

Enhanced inhibitory neurotransmission in the cerebellar cortex of Atp1a3-deficient heterozygous mice

Dystonia is characterized by excessive involuntary and prolonged simultaneous contractions of both agonist and antagonist muscles. Although the basal ganglia have long been proposed as the primary region, recent studies indicated that the cerebellum also plays a key role in the expression of dystonia. One hereditary form of dystonia, rapid-onset dystonia with parkinsonism (RDP), is caused by loss of function mutations of the gene for the Na pump α3 subunit (ATP1A3). Little information is available on the affected brain regions and mechanism for dystonia by the mutations in RDP. The Na pump is composed of α and β subunits and maintains ionic gradients of Na(+) and K(+) across the cell membrane. The gradients are utilized for neurotransmitter reuptake and their alteration modulates neural excitability. To provide insight into the molecular aetiology of RDP, we generated and analysed knockout heterozygous mice (Atp1a3(+/-)). Atp1a3(+/-) showed increased symptoms of dystonia that is induced by kainate injection into the cerebellar vermis. Atp1a3 mRNA was highly expressed in Purkinje cells and molecular-layer interneurons, and its product was concentrated at Purkinje cell soma, the site of abundant vesicular γ-aminobutyric acid transporter (VGAT) signal, suggesting the presynaptic localization of the α3 subunit in the inhibitory synapse. Electrophysiological studies showed that the inhibitory neurotransmission at molecular-layer interneuron-Purkinje cell synapses was enhanced in Atp1a3(+/-) cerebellar cortex, and that the enhancement originated via a presynaptic mechanism. Our results shed light on the role of Atp1a3 in the inhibitory synapse, and potential involvement of inhibitory synaptic dysfunction for the pathophysiology of dystonia.

Capillary endothelial Na(+), K(+), ATPase transporter homeostasis and a new theory for migraine pathophysiology

BACKGROUND

Cerebrospinal fluid sodium concentration ([Na(+)](csf)) increases during migraine, but the cause of the increase is not known.

OBJECTIVE

Analyze biochemical pathways that influence [Na(+)](csf) to identify mechanisms that are consistent with migraine.

METHOD

We reviewed sodium physiology and biochemistry publications for links to migraine and pain.

RESULTS

Increased capillary endothelial cell (CEC) Na(+), K(+), -ATPase transporter (NKAT) activity is probably the primary cause of increased [Na(+)](csf). Physiological fluctuations of all NKAT regulators in blood, many known to be involved in migraine, are monitored by receptors on the luminal wall of brain CECs; signals are then transduced to their abluminal NKATs that alter brain extracellular sodium ([Na(+)](e)) and potassium ([K(+)](e)).

CONCLUSIONS

We propose a theoretical mechanism for aura and migraine when NKAT activity shifts outside normal limits: (1) CEC NKAT activity below a lower limit increases [K(+)](e), facilitates cortical spreading depression, and causes aura; (2) CEC NKAT activity above an upper limit elevates [Na(+)](e), increases neuronal excitability, and causes migraine; (3) migraine-without-aura may arise from CEC NKAT over-activity without requiring a prior decrease in activity and its consequent spreading depression; (4) migraine triggers disturb, and treatments improve, CEC NKAT homeostasis; (5) CEC NKAT-induced regulation of neural and vasomotor excitability coordinates vascular and neuronal activities, and includes occasional pathology from CEC NKAT-induced apoptosis or cerebral infarction.

Sodium/potasium ATPase (Na+, K+-ATPase) and ouabain/related cardiac glycosides: a new paradigm for development of anti- breast cancer drugs?

Prolonged exposure to 17beta-estradiol (E2) is a key etiological factor for human breast cancer. The biological effects and carcinogenic effects of E2 are mediated via estrogen receptors (ERs), ERalpha and ERbeta. Anti-estrogens, e.g. tamoxifen, and aromatase inhibitors have been used to treat ER-positive breast cancer. While anti-estrogen therapy is initially successful, a major problem is that most tumors develop resistance and the disease ultimately progresses, pointing to the need of developing alternative drugs targeting to other critical targets in breast cancer cells. We have identified that Na+, K+-ATPase, a plasma membrane ion pump, has unique/valuable properties that could be used as a potentially important target for breast cancer treatment: (a) it is a key player of cell adhesion and is involved in cancer progression; (b) it serves as a versatile signal transducer and is a target for a number of hormones including estrogens and (d) its aberrant expression and activity are implicated in the development and progression of breast cancer. There are several lines of evidence indicating that ouabain and related digitalis (the potent inhibitors of Na+, K+-ATPase) possess potent anti-breast cancer activity. While it is not clear how the suggested anti-cancer activity of these drugs work, several observations point to ouabain and digitalis as being potential ER antagonists. We critically reviewed many lines of evidence and postulated a novel concept that Na+, K+-ATPase in combination with ERs could be important targets of anti-breast cancer drugs. Modulators, e.g. ouabain and related digitalis could be useful to develop valuable anti-breast cancer drugs as both Na+, K+-ATPase inhibitors and ER antagonists.

Age-related changes in cyclic GMP and PKG-stimulated cerebellar Na,K-ATPase activity

Energy deficiency and dysfunction of the Na,K-ATPase are common consequences of many pathological insults. Glutamate through cyclic GMP and cyclic GMP-dependent protein kinase (PKG) has been shown to stimulate alpha(2/3)-Na,K-ATPase activity in the central nervous system. Thus, a slight impairment of this pathway may amplify the disruption of ion homeostasis in the presence of a non-lethal insult. We investigate the effect of aging (4, 12 and 24 months) on the glutamate-cyclic GMP-PKG modulation of alpha1, alpha(2/3)-Na,K-ATPase activity in rat cerebellum and the stimulation of the glutamate-cyclic GMP-PKG pathway at different levels. Cyclic GMP levels and alpha(2/3)-Na,K-ATPase activity were progressively decreased from 4 and 24 month-old animals. However, PKG basal activity was reduced between 4 and 12 months, and no additional change was observed at 24 months. The ability of 8-Br-cyclic GMP to stimulate PKG activity was only reduced between 12 and 24 months. Moreover, glutamate or 8-Br-cyclic GMP promoted a smaller increase of alpha(2/3)-Na,K-ATPase activity at 24 months, when compared to 4 and 12 months. In spite of the age-related reduced basal levels of cyclic GMP, the production induced by CO or NO was not age-related. Finally, inhibition of PKG activation by KT5823 revealed a lower sensitivity of the enzyme at the older age. Taken together, these data show that basal age-related decline in sodium pump activity is a consequence of changes in different steps of the cyclic GMP-PKG pathway. On the other hand, age-related reduction in glutamate positive modulation of cerebellar alpha(2/3)-Na,K-ATPase is linked to a defective PKG signaling pathway.

Depletion of glial cell line-derived neurotrophic factor in substantia nigra neurons of Parkinson's disease brain

The distribution of nerve growth factor (NGF), ciliary neurotrophic factor (CNTF), glial cell line-derived neurotrophic factor (GDNF), brain derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4) in substantia nigra pars compacta (SNc) of Parkinson's disease (PD) brains was investigated by immunofluorescence. Cases studied included four 69-77 year old neurologically normal male controls and four 72-79 year old male PD patients. Integrated optical densities (IODs) of immunofluorescence over individual neuromelanin-containing neurons and in areas of neuropil and the number of neurons on H & E stained adjacent sections were quantitated with the use of the BioQuant Image Analyzer. Data were statistically analyzed by ANOVA, including the unpaired two-tailed Student t-test and the Mann-Whitney test. The results showed 55.8% (P<0.0001) dropout of SNc neurons in PD brains compared to age-matched controls. Despite considerable neuronal dropout, immunofluorescent NTFs in the PD brains showed differential reductions that were consistent within the group as compared to age-matched controls: reductions were GDNF, 19.4%/neuron (P<0.0001), 20.2%/neuropil (P<0.0001); CNTF, 11.1%/neuron (P<0.0001), 9.4%/neuropil (P<0.0001); BDNF, 8.6%/neuron (P<0.0001), 2.5%/neuropil. NGF, NT-3 and NT-4 showed no significant differences within surviving neurons or neuropil. Since the depletion of GDNF both within surviving neurons and neuropil was twice as great as that of CNTF and BDNF and since the other NTFs showed no changes, GDNF, of the tested NTFs, is probably the most susceptible and the earliest to decrease in the surviving neurons of SNc. These observations suggest a role for decreased availability of GDNF in the process of SNc neurodegeneration in PD.

Neuronal interaction determines the expression of the alpha-2 isoform of Na, K-ATPase in oligodendrocytes

Na,K-ATPase is an integral membrane enzyme responsible for maintenance of the transmembrane Na+/K+ gradient which generates membrane excitability. Previous studies showed that oligodendrocytes within the CNS robustly expressed the alpha2 isoform of the Na,K-ATPase while oligodendrocytes in isolated cultures did not. We tested whether the levels of this isoform might be modulated by interactions with neurons. Western blots showed alpha2 protein expression was very low in rat optic nerve immediately after birth, but that expression was greatly increased by days 5 and 14. In adult optic nerves, levels were barely detectable. Since the first myelinated axons are observed in rat optic nerve at day 5, and the next 2 weeks are considered the period of peak myelination, this timing suggested a relationship between oligodendrocyte-neuron contact, myelination onset and the upregulation of the alpha2 isoform. In further experiments we plated oligodendrocytes in isolation or in co-culture with neurons dissociated from cerebral cortex at the day of birth. After 6 days in vitro, 45% of oligodendrocytes co-cultured with neurons expressed abundant alpha2 protein which was detected by immunohistochemistry, a six-fold increase over cells expressing alpha2 protein in isolated cultures. Conditioned medium from neuronal cultures did not affect alpha2 levels in oligodendrocytes. These results suggest that neurons may play a role in upregulating glial expression of the alpha2 isoform during peak periods of myelination, and that the effect is likely to be dependent on contact.

Na,K-ATPase mRNA levels and plaque load in Alzheimer's disease

The expression of Na,K-ATPase alpha 1- and alpha 3-mRNAs was analyzed by in situ hybridization in the superior frontal cortex and cerebellum of brains from five Alzheimer's disease (AD), five nondemented age-matched, and three young control subjects. Brains with well-preserved RNA, tested by Northern hybridization of immobilized RNA with [32P]-labeled human beta-actin riboprobe, were chosen for analysis. In situ hybridization was performed on formalin-fixed, 5 microns-thick Paraplast sections with [35S]-labeled riboprobes prepared by in vitro transcription of the respective linearized clones: a 537-bp EcoRI-PstI fragment of alpha 1-cDNA and a 342-bp PstI-EcoRI fragment of alpha 3-cDNA. In cortex, grains related to mRNA were measured by density per unit area in five cortical columns separated by 1.0-1.2 cm in each of two adjacent sections. Each cortical column of 180-micron width was divided into four depths orthogonal to the pial surface between the pia and the white matter. Amyloid plaques were counted in the same regions of adjacent sections. In addition, alpha 3-mRNA grain clusters over individual pyramidal neurons within depth 4 were analyzed. We found the following significant changes (p < 0.05): 1. Increases in total alpha 1-mRNA by 13-19% in AD compared to young and by 7-12% in AD compared to age-matched controls. 2. Decrease in total alpha 3-mRNA by 31-38% in AD compared to young and age-matched controls. 3. Decrease in alpha 3-mRNA content over individual pyramidal perikarya by 14% in normal aged brains without plaques compared to young controls, and by 44% in AD relative to young controls and by 35% compared to age-matched controls. No significant difference (p < 0.2) was found with respect to alpha 1- or alpha 3-mRNA in cerebellar cortex or individual Purkinje cells among any of the groups. In addition, there was a trend toward an inverse correlation between the levels of alpha 3-mRNA and of diffuse plaques, but not of neuritic plaques, in AD cases.

IN CONCLUSION

1. The increases in alpha 1-mRNA in AD may be related to an increased reactive gliosis. 2. The declines in alpha 3-mRNA per individual neuron found in normal aging occur prior to the formation of diffuse plaques and are greatly accelerated in AD. 3. The declines in alpha 3-mRNA per neuron found in normal aging may predispose to or potentiate AD pathogenesis.