Na(+)-K(+)-ATPase gene expression during in vitro development of rat fetal forebrain

Na+/K+-ATPase Alpha 2 Isoform Elicits Rac1-Dependent Oxidative Stress and TLR4-Induced Inflammation in the Hypothalamic Paraventricular Nucleus in High Salt-Induced Hypertension

Numerous studies have indicated that a high salt diet inhibits brain Na⁺/K⁺-ATPase (NKA) activity, and affects oxidative stress and inflammation in the paraventricular nucleus (PVN). Furthermore, Na⁺/K⁺-ATPase alpha 2-isoform (NKA α2) may be a target in the brain, taking part in the development of salt-dependent hypertension. Therefore, we hypothesized that NKA α2 regulates oxidative stress and inflammation in the PVN in the context of salt-induced hypertension. Part I: We assessed NKA subunits (NKA α1, NKA α2, and NKA α3), Na⁺/K⁺-ATPase activity, oxidative stress, and inflammation in a high salt group (8% NaCl) and normal salt group (0.3% NaCl). Part II: NKA α2 short hairpin RNA (shRNA) was bilaterally microinjected into the PVN of salt-induced hypertensive rats to knockdown NKA α2, and we explored whether NKA α2 regulates downstream signaling pathways related to protein kinase C γ (PKC γ)-dependent oxidative stress and toll-like receptor 4 (TLR4)-induced inflammation in the PVN to promote the development of hypertension. High salt diet increased NKA α1 and NKA α2 protein expression in the PVN but had no effect on NKA α3 compared to the normal salt diet. Na⁺/K⁺-ATPase activity and ADP/ATP ratio was lower, but NAD(P)H activity and NF-κB activity in the PVN were higher after a high salt diet. Bilateral PVN microinjection of NKA α2 shRNA not only improved Na⁺/K⁺-ATPase activity and ADP/ATP ratio but also suppressed PKC γ-dependent oxidative stress and TLR4-dependent inflammation in the PVN, thus decreasing sympathetic activity in rats with salt-induced hypertension. NKA α2 in the PVN elicits PKC γ/Rac1/NAD (P)H-dependent oxidative stress and TLR4/MyD88/NF-κB-induced inflammation in the PVN, thus increasing MAP and sympathetic activity during the development of salt-induced hypertension.

Lead acetate toxicity in vitro: Dependence on the cell composition of the cultures

It is well known that exposure to low doses of lead causes long-lasting neurobehavioural deficits, but the cellular changes underlying these behavioural changes remain to be elucidated. A protective role of glial cells on neurons through lead sequestration by astrocytes has been proposed. The possible modulation of lead neurotoxicity by neuron-glia interactions was examined in three-dimensional cultures of foetal rat telencephalon. Mixed-brain cell cultures or cultures enriched in either neurons or glial cells were treated for 10 days with lead acetate (10(-6) m), a concentration below the limit of cytotoxicity. Intracellular lead content and cell type-specific enzyme activities were determined. It was found that in enriched cultures neurons stored more lead than glial cells, and each cell type alone stored more lead than in co-culture. Moreover, glial cells but not neurons were more affected by lead in enriched culture than in co-culture. These results show that neuron-glia interactions attenuate the cellular lead uptake and the glial susceptibility to lead, but they do not support the idea of a protective role of astrocytes.

Evaluation of aggregating brain cell cultures for the detection of acute organ-specific toxicity

As part of the ACuteTox project aimed at the development of non-animal testing strategies for predicting human acute oral toxicity, aggregating brain cell cultures (AGGR) were examined for their capability to detect organ-specific toxicity. Previous multicenter evaluations of in vitro cytotoxicity showed that some 20% of the tested chemicals exhibited significantly lower in vitro toxicity as expected from in vivo toxicity data. This was supposed to be due to toxicity at supracellular (organ or system) levels. To examine the capability of AGGR to alert for potential organ-specific toxicants, concentration-response studies were carried out in AGGR for 86 chemicals, taking as endpoints the mRNA expression levels of four selected genes. The lowest observed effect concentration (LOEC) determined for each chemical was compared with the IC20 reported for the 3T3/NRU cytotoxicity assay. A LOEC lower than IC20 by at least a factor of 5 was taken to alert for organ-specific toxicity. The results showed that the frequency of alerts increased with the level of toxicity observed in AGGR. Among the chemicals identified as alert were many compounds known for their organ-specific toxicity. These findings suggest that AGGR are suitable for the detection of organ-specific toxicity and that they could, in conjunction with the 3T3/NRU cytotoxicity assay, improve the predictive capacity of in vitro toxicity testing.

Enhanced pressor response to increased CSF sodium concentration and to central ANG I in heterozygous alpha2 Na+ -K+ -ATPase knockout mice

Intracerebroventricular (ICV) infusion of NaCl mimics the effects of a high-salt diet in salt-sensitive hypertension, raising the sodium concentration in the cerebrospinal fluid (CSF [Na]) and subsequently increasing the concentration of an endogenous ouabain-like substance (OLS) in the brain. The OLS, in turn, inhibits the brain Na(+)-K(+)-ATPase, causing increases in the activity of the brain renin-angiotensin system (RAS) and blood pressure. The Na(+)-K(+)-ATPase alpha (catalytic)-isoform(s) that mediates the pressor response to increased CSF [Na] is unknown, but it is likely that one or more isoforms that bind ouabain with high affinity are involved (e.g., the Na(+)-K(+)-ATPase alpha(2)- and/or alpha(3)-subunits). We hypothesize that OLS-induced inhibition of the alpha(2)-subunit mediates this response. Therefore, a chronic reduction in alpha(2) expression via a heterozygous gene knockout (alpha(2) +/-) should enhance the pressor response to increased CSF [Na]. Intracerebroventricular (ICV) infusion of artificial CSF containing 0.225 M NaCl increased mean arterial pressure (MAP) in both wild-type (+/+) and alpha(2) +/- mice, but to a greater extent in alpha(2) +/-. Likewise, the pressor response to ICV ouabain was enhanced in alpha(2) +/- mice, demonstrating enhanced sensitivity to brain Na(+)-K(+)-ATPase inhibition per se. The pressor response to ICV ANG I but not ANG II was also enhanced in alpha(2) +/- vs. alpha(2)+/+ mice, suggesting an enhanced brain RAS activity that may be mediated by increased brain angiotensin converting enzyme (ACE). The latter hypothesis is supported by enhanced ACE ligand binding in the organum vasculosum laminae terminalis. These studies demonstrate that chronic downregulation of Na(+)-K(+)-ATPase alpha(2)-isoform expression by heterozygous knockout increases the pressor response to increased CSF [Na] and activates the brain RAS. Since these changes mimic those produced by the endogenous brain OLS, the brain alpha(2)-isoform may be a target for the brain OLS during increases in CSF [Na], such as in salt-dependent hypertension.

Endogenous brain Na pumps, brain ouabain-like substance and the alpha2 isoform in salt-dependent hypertension

An endogenous ouabain-like substance (OLS) plays a critical role in the etiology of experimental models of human hypertension induced by a high salt diet. Early on, evidence for a role of this Na, K-ATPase inhibitor in blood pressure regulation was provided mainly by correlations of blood pressure with the levels of circulating Na, K-ATPase inhibitor. However, over the past decade, numerous studies have shown that endogenous Na pump inhibitors in the brain mediate salt-dependent hypertension in a variety of experimental models, including Dahl salt-sensitive (Dahl-S) and spontaneously hypertensive (SHR) rats on a high-salt diet. Other forms of hypertension that are known to be mediated by endogenous ouabain-like substances include steroid/salt- (e.g., DOCA-salt) and ACTH-induced hypertension. Even when exogenous ouabain is peripherally administered and/or the plasma ouabain/OLS level is increased in rats, the resulting hypertension is of CNS origin. After peripheral ouabain administration, ouabain levels increase in the plasma and the inhibitor subsequently accumulates in the brain. The ensuing hypertension is abolished by the intracerebroventricular (icv) administration of an anti-ouabain antibody (but not by the same antibody dose given iv), by discrete excitotoxic lesions in the brain or by ganglionic blockade, demonstrating that the response is neurally mediated. The pressor response to stimuli that increase the brain OLS (high salt diet, icv sodium) or to icv ouabain is abolished by icv losartan, demonstrating that the brain OLS activates the brain renin-angiotensin system (RAS) downstream. There are three isoforms of the catalytic alpha subunit of the Na, K-ATPase in the brain and cardiovascular system (alpha1, alpha2 and alpha3), but it is not known which brain isoform(s) mediate the hypertensive effects of circulating/CNS ouabain. Preliminary studies in gene-targeted mice suggest that the alpha2 isoform plays a critical role.

Programmed aortic dysfunction and reduced Na+,K+-ATPase activity present in first generation offspring of lard-fed rats does not persist to the second generation

We have previously reported that male and female offspring of Sprague-Dawley rats fed a diet rich (approximately 50% of caloric intake from fat) in animal fat (lard) during pregnancy and suckling (OHF) demonstrate cardiovascular dysfunction, including blunted endothelium-dependent vasodilatation in the aorta as well as reduced renal Na(+),K(+)-ATPase activity. Cardiovascular dysfunction has been reported in other models of developmental programming and some researchers describe transmission from F(1) to F(2) generations. Here we report a study of vascular function, as assessed in isolated rings of aorta mounted in an organ bath, and renal Na(+),K(+)-ATPase activity in 6-month-old male and female F(2) offspring of lard-fed and control-fed (OC) dams (n = 13 per diet group). An increase in brain (OC 0.61 +/- 0.01% versus OHF 0.66 +/- 0.02% of bodyweight) and kidney weights (OC 0.32 +/- 0.01% versus OHF 0.37 +/- 0.01% of bodyweight) was observed in female F(2) offspring of lard-fed dams compared with F(2) controls (P < 0.03). Constrictor responses to phenylephrine in the aorta were not different from F(2) controls (repeated measures ANOVA, P = 0.85). Also, endothelium-dependent dilator function, as assessed by responses to acetylcholine (repeated measures ANOVA, P = 0.96) and passive distensibility in the absence of extracellular calcium (repeated measures ANOVA, P = 0.68), was similar. Additionally, renal Na(+),K(+)-ATPase activity was not statistically different from that observed in control animals (ANOVA, P = 0.89). Although a maternal diet rich in animal fat has deleterious effects on parameters of cardiovascular risk in F(1) animals, it does not appear that disorders previously reported in the F(1) generation are transmitted to the F(2) generation.

Characterization of long-term mouse brain aggregating cultures: Evidence for maintenance of neural precursor cells

An extensive characterization of fetal mouse brain cell aggregates has been performed using immunohistochemical and stereological methods. Single cell suspensions from mechanically dissociated cortex and hippocampus were cultured in serum-free, B27-supplemented medium under constant gyratory agitation for up to 56 days. Three-dimensional aggregates started to form immediately after seeding and reached a final average size of 500 microm in diameter. Among the cell types identified, neurons were the most abundant cells in the aggregates, followed by astrocytes, microglia, and oligodendrocytes. Western blotting for synaptophysin and immunostaining for neurotransmitter-related molecules indicated the presence of well-defined phenotypic characteristics of the neurons in this culture system, suggesting functionality. Proliferating cells, many with neural precursor cell properties, were seen throughout the culture period and could be isolated from the aggregates even after 2 months in culture. Neural precursor cells were isolated from the aggregates after more than 1 month in culture; these cells were successfully differentiated into neurons, astrocytes, and oligodendrocytes. The aggregate culture system may provide a versatile tool for molecular dissection of processes identified in mouse models, including transgenic animals and manipulation of neural precursor cells.

Aggregating neural cell cultures

When freshly dissociated embryonic tissues are kept under gyratory agitation, the cells aggregate to form three-dimensional spheroids in which the cells can migrate and organize themselves, attaining maximal cellular differentiation after weeks of culture. The three-dimensional architecture of the aggregates permits direct cell-to-cell interactions and the formation of a natural cell matrix, which is fundamental to the acquisition of the histotypic properties of the aggregates. This unit describes protocols for preparing forebrain cells from embryonic rodents for aggregating cultures and maintaining these cultures to the differentiated state.

The Na,K-ATPase alpha 2 isoform is expressed in neurons, and its absence disrupts neuronal activity in newborn mice

Na,K-ATPase is an ion transporter that impacts neural and glial physiology by direct electrogenic activity and the modulation of ion gradients. Its three isoforms in brain have cell-type and development-specific expression patterns. Interestingly, our studies demonstrate that in late gestation, the alpha2 isoform is widely expressed in neurons, unlike in the adult brain, in which alpha2 has been shown to be expressed primarily in astrocytes. This unexpected distribution of alpha2 isoform expression in neurons is interesting in light of our examination of mice lacking the alpha2 isoform which fail to survive after birth. These animals showed no movement; however, defects in gross brain development, muscle contractility, neuromuscular transmission, and lung development were ruled out. Akinesia suggests a primary neuronal defect and electrophysiological recordings in the pre-Bötzinger complex, the brainstem breathing center, showed reduction of respiratory rhythm activity, with less regular and smaller population bursts. These data demonstrate that the Na,K-ATPase alpha2 isoform could be important in the modulation of neuronal activity in the neonate.

Thyroidal regulation of different isoforms of NaKATPase in the primary cultures of neurons derived from fetal rat brain

The developmental profile of the different isoforms of NaKATPase have been investigated using primary cultures of isolated neurons initiated from 17 day old fetal rat brain. Northern blot analysis showed that the expression of three alpha isoforms (alpha(1), alpha(2) and alpha(3)) and two beta isoforms (beta(1) and beta(2)) increased progressively and reached a peak between 12 to 16 days of culture. Comparison of the mRNA levels of these isoforms in the cells maintained in thyroid hormone deficient (TH def) and thyroid hormone supplemented (TH sup) media for 6-12 days, revealed for the first time that in the neurons three alpha and two beta isoforms of NaKATPase are sensitive to TH. Furthermore immunocytochemical staining of these cells with isoform specific NaKATPase antibodies showed that the uniform distribution of alpha(2), alpha(3) and beta(2) isoforms in the neuronal processes require the presence of TH. These results establish neurons as the target cells for the regulation of NaKATPase by TH in the developing brain.

Potassium recycling pathways in the human cochlea

OBJECTIVES/HYPOTHESIS

Potential pathways for recycling potassium (K+) used in the maintenance of inner ear electrochemical gradients have been elucidated in animal models. However, little is known about K+ transport in the human cochlea. This study was designed to characterize putative K+ recycling pathways in the human ear and to determine whether observations from animal models can be extrapolated to humans.

STUDY DESIGN

A prospective laboratory study using an immunohistochemical approach to analyze the distribution of key ion transport mediators in the human cochlea.

METHODS

Human temporal bones were fixed in situ within 1 to 6 hours of death and subsequently harvested at autopsy. Decalcification was accomplished with the aid of microwaving. Immunohistochemical staining was then performed to define the presence and cell type-specific distribution of Na,K-ATPase, sodium-potassium-chloride cotransporter (NKCC), and carbonic anhydrase (CA) in the inner ear.

RESULTS

Staining patterns visualized in the human cochlea closely paralleled those seen in other species. Anti-Na,K-ATPase stained strongly the basolateral plasma membrane of strial marginal cells and nerve endings underlying hair cells. This antibody also localized Na,K-ATPase to type II, type IV, and type V fibrocytes in the spiral ligament and in limbal fibrocytes. NKCC was present in the basolateral membrane of strial marginal cells as well as in type II, type V, and limbal fibrocytes. Immunoreactive carbonic anhydrase was present in type I and type III fibrocytes and in epithelial cells lining Reissner's membrane and the spiral prominence.

CONCLUSIONS

The distribution of several major ion transport proteins in the human cochlea is similar but not identical to that described in various rodent models. These results support the presence of a complex system for recycling and regulating K+ homeostasis in the human cochlea, similar to that described in other mammalian species.

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.

Expression, activity and distribution of Na,K-ATPase subunits during in vitro neuronal induction

The expression pattern of the alpha and beta isoforms and the gamma subunit of the Na,K-ATPase was investigated during in vitro induction of pluripotent murine embryonic stem (ES) cells into neuronal cells. alpha1 protein was expressed in undifferentiated ES (UES) cells and throughout all stages studied. In contrast, alpha3 protein was prominent only when neuronal cells have reached full differentiation. In this model, neuron-depleted cultures did not express the alpha3 isoform, indicating its specificity for mature neuronal cells. UES possessed Na,K-ATPase activity consistent with a single isoform (alpha1), whereas in fully mature neuronal cells a ouabain-sensitive isoform (alpha3) accounted for 27+/-4% of the activity, and a ouabain-resistant isoform (alpha1) 66+/-3%. Immunocytochemistry of mature neuronal cells for alpha1 and alpha3 proteins showed a similar distribution, including cell soma and processes, without evidence of polarization. beta1 protein was expressed in uninduced ES, embryonic bodies (EB) and neuronal cells. While proteins of the beta2 and beta3 isoforms were not detected by immunoblots (except for beta2 in UES), their mRNAs were detected in UES and EB (beta2 and beta3), and in immature and fully differentiated neuronal cells (beta3). Message for the beta2 isoform, however, was not present in neuronal cells. gamma subunit mRNA and protein were undetectable at any stage. These results provide further characterization of neuron-like cells obtained by induction of ES cells in vitro, and establish a model for the expression of isoforms of the Na,K-ATPase during neuronal differentiation. The relation to other aspects of neuronal cell development and relevance to a specialised function for the alpha3 subunit in neurons are discussed.

Oligodendrocytes in brain and optic nerve express the beta3 subunit isoform of Na,K-ATPase

The Na,K-ATPase, which catalyzes the active transport of Na(+) and K(+), has two principal subunits (alpha and beta) that have several genetically distinct isoforms. Most of these isoforms are expressed in the nervous system, but certain ones are preferentially expressed in glia and others in neurons. Of the beta isoforms, beta1 predominates in neurons and beta2 in astrocytes, although there are some exceptions. Here we demonstrate that beta3 is expressed in rat and mouse white matter oligodendrocytes. Immunofluorescence microscopy identified beta3 in oligodendrocytes of rat brain white matter in typical linear arrays of cell bodies between fascicles of axons. The intensity of stain peaked at 20 postnatal days. beta3 was identified in cortical oligodendrocytes grown in culture, where it was expressed in processes and colocalized with antibody to galactocerebroside. In the mouse and rat optic nerve, beta3 stain was seen in oligodendrocytes, where it colocalized with carbonic anhydrase II. For comparison, optic nerve was stained for the beta1 and beta2 subunits, showing distinct patterns of labelling of axons (beta1) and astrocytes (beta2). The C6 glioma cell line was also found to express the beta3 isoform preferentially. Since beta3 was not found at detectable levels in astrocytes, this suggests that C6 is closer to oligodendrocytes than astrocytes in the glial cell lineage.

Differentiation of rat striatal embryonic stem cells in vitro: monolayer culture vs. three-dimensional coculture with differentiated brain cells

Several groups have demonstrated the existence of self-renewing stem cells in embryonic and adult mouse brain. In vitro, these cells proliferate in response to epidermal growth factor, forming clusters of nestin-positive cells that may be dissociated and subcultured repetitively. Here we show that, in stem cell clusters derived from rat embryonic striatum, cell proliferation decreased with increasing number of passages and in response to elevated concentrations of potassium (30 mM KCl). In monolayer culture, the appearance of microtubule-associated protein type-5-immunoreactive (MAP-5(+)) cells (presumptive neurons) in response to basic fibroblast growth factor (bFGF) was reduced at low cell density and with increasing number of passages. In the presence of bFGF, elevated potassium caused a more differentiated neuronal phenotype, characterized by an increased proportion of MAP-5(+) cells, extensive neuritic branching, and higher specific activity of glutamic acid decarboxylase. Dissociated stem cells were able to invade cultured brain cell aggregates containing different proportions of neurons and glial cells, whereas they required the presence of a considerable proportion of glial cells in the host cultures to become neurofilament H-positive. The latter observation supports the view that astrocyte-derived factors influence early differentiation of the neuronal cell lineage.

Peroxisome proliferator-activated receptor beta regulates acyl-CoA synthetase 2 in reaggregated rat brain cell cultures

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that regulate the expression of many genes involved in lipid metabolism. The biological roles of PPARalpha and PPARgamma are relatively well understood, but little is known about the function of PPARbeta. To address this question, and because PPARbeta is expressed to a high level in the developing brain, we used reaggregated brain cell cultures prepared from dissociated fetal rat telencephalon as experimental model. In these primary cultures, the fetal cells initially form random aggregates, which progressively acquire a tissue-specific pattern resembling that of the brain. PPARs are differentially expressed in these aggregates, with PPARbeta being the prevalent isotype. PPARalpha is present at a very low level, and PPARgamma is absent. Cell type-specific expression analyses revealed that PPARbeta is ubiquitous and most abundant in some neurons, whereas PPARalpha is predominantly astrocytic. We chose acyl-CoA synthetases (ACSs) 1, 2, and 3 as potential target genes of PPARbeta and first analyzed their temporal and cell type-specific pattern. This analysis indicated that ACS2 and PPARbeta mRNAs have overlapping expression patterns, thus designating the ACS2 gene as a putative target of PPARbeta. Using a selective PPARbeta activator, we found that the ACS2 gene is transcriptionally regulated by PPARbeta, demonstrating a role for PPARbeta in brain lipid metabolism.

Separate neuronal and glial Na+,K+-ATPase isoforms regulate glucose utilization in response to membrane depolarization and elevated extracellular potassium

The role of cell type-specific Na+,K+-ATPase isozymes in function-related glucose metabolism was studied using differentiated rat brain cell aggregate cultures. In mixed neuron-glia cultures, glucose utilization, determined by measuring the rate of radiolabeled 2-deoxyglucose accumulation, was markedly stimulated by the voltage-dependent sodium channel agonist veratridine (0.75 micromol/L), as well as by glutamate (100 micromol/L) and the ionotropic glutamate receptor agonist N-methyl-D-aspartate (NMDA) (10 micromol/L). Significant stimulation also was elicited by elevated extracellular potassium (12 mmol/L KCl), which was even more pronounced at 30 mmol/L KCl. In neuron-enriched cultures, a similar stimulation of glucose utilization was obtained with veratridine, specific ionotropic glutamate receptor agonists, and 30 mmol/L but not 12 mmol/L KCl. The effects of veratridine, glutamate, and NMDA were blocked by specific antagonists (tetrodotoxin, CNQX, or MK801, respectively). Low concentrations of ouabain (10(-6) mol/L) prevented stimulation by the depolarizing agents but reduced only partially the response to 12 mmol/L KCl. Together with previous data showing cell type-specific expression of Na+,K+-ATPase subunit isoforms in these cultures, the current results support the view that distinct isoforms of Na+,K+-ATPase regulate glucose utilization in neurons in response to membrane depolarization, and in glial cells in response to elevated extracellular potassium.

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.

Isoforms of Na,K-ATPase alpha and beta subunits in the rat cerebellum and in granule cell cultures

There are multiple isoforms of the Na,K-ATPase in the nervous system, three isoforms of the alpha subunit, and at least two of the beta subunit. The alpha subunit is the catalytic subunit. The beta subunit has several roles. It is required for enzyme assembly, it has been implicated in neuron-glia adhesion, and the experimental exchange of beta subunit isoforms modifies enzyme kinetics, implying that it affects functional properties. Here we describe the specificities of antibodies against the Na,K-ATPase beta subunit isoforms beta1 and beta2. These antibodies, along with antibodies against the alpha subunit isoforms, were used to stain sections of the rat cerebellum and cultures of cerebellar granule cells to ascertain expression and subcellular distribution in identifiable cells. Comparison of alpha and beta isoform distribution with double-label staining demonstrated that there was no preferential association of particular alpha subunits with particular beta subunits, nor was there an association with excitatory or inhibitory neurotransmission modes. Isoform composition differences were seen when Purkinje, basket, and granule cells were compared. Whether beta1 and beta2 are specific for neurons and glia, respectively, has been controversial, but expression of both beta subunit types was seen here in granule cells. In rat cerebellar astrocytes, in sections and in culture, alpha2 expression was prominent, yet the expression of either beta subunit was low in comparison. The complexity of Na,K-ATPase isoform distribution underscores the subtlety of its regulation and physiological role in excitable cells.

Differential effects of hypothyroidism on Na-K-ATPase mRNA alpha isoforms in the developing rat brain

In the developing rat cerebrum, the level of different isoforms of Na-K-ATPase mRNA increases significantly during the first three postnatal weeks, which represent the critical period of synaptogenesis and myelination-the two thyroid hormone-sensitive maturational events. To determine the possible functional relationship of these isoforms with maturational events in the developing brain and their mode of regulation by T3, we have examined the effect of hypothyroidism on the expression of the different alpha-isoforms (alpha 1, alpha 2, and alpha 3) of Na-K-ATPase mRNA covering the first 3 wk of postnatal development. Quantitation of these mRNAs from cerebra of 1-, 5-, 10-, 15-, and 20-d-old normal and hypothyroid rats by Northern blot analysis indicate that alpha 3 mRNA is not only predominantly expressed throughout this entire period of study but also represents the species which is most severely affected in the hypothyroid brain. The relative sensitivity for the expression of these mRNAs to T3 were alpha 3 > alpha 1 > alpha 2. These results, together with the report of predominant expression of the alpha 3 isoform in neuronal cells, suggest specific functional involvement of this isoform with the decisive maturational events in the rat brain. Kinetic studies on in vivo induction of Na-K-ATPase alpha-mRNAs by T3 in the 15-d-old hypothyroid rat shows clear stimulation of all the isoforms within 1 h of the administration of the optimal dose (200 micrograms T3/100 g body wt) suggesting a direct, possibly transcriptional effect of the hormone on the expression of these genes.

Developmental changes of K(+)-dependent para-nitrophenylphosphatase (Na(+)-K(+)-ATPase) distribution in the synaptic regions in the cerebral cortex of rats

Using Mayahara's method, the distribution of K(+)-dependent p-nitrophenylphosphatase activity was examined electron microscopically in the synaptic regions of the cerebral cortex of 10, 15 and 60-day-old Wistar rats. The enzyme achieved gradually its characteristic localization and uniform distribution. The main developmental changes were associated with the establishment of the postsynaptic density's activity. The controls with ouabain revealed activity only on the postsynaptic densities.

Expression of interleukin-3 and tumor necrosis factor-beta mRNAs in cultured microglia

The function of interleukin-3 (or multi-CSF) in the hemopoietic system has been studied in great detail. Although its growth promoting activity on brain microglial cells has been confirmed both in vitro and in vivo, its presence in the brain and even in cultured brain cells has repeatedly been questioned. We have shown recently that isolated rat microglia express mRNA(IL-3) and synthesize IL-3 polypeptide. It is shown here by use of the PCR method, that mRNA(IL-3) is found also in C6 glioblastoma, in rat aggregate cultures, and in newborn and adult rat brain. Quantitation of amplified cDNA(IL-3) was achieved by non-competitive RT-PCR using an elongated internal standard. IL-3 messenger RNA was almost undetectable in vivo and low in (serum-free) aggregate cultures. In isolated microglia, mRNA(IL-3) was increased upon treatment with LPS, PHA, with the cytokines IL-1 or TNF-alpha, with retinoic acid, dbcAMP or the phorbol ester TPA. Effects of LPS were inhibited by dexamethasone, while the glucocorticoid by itself had no effect on basal IL-3 expression. LPS increased mRNA(IL-3) in a concentration-dependent manner beginning with 10 pg/ml and reaching plateau levels at 10 ng/ml. LPS also increased mRNAs of TNF-alpha and TNF-beta. TNF-alpha mRNA was already detectable in untreated microglia and LPS-increased levels were sustained for a few days. In contrast, TNF-beta mRNA was observed only between 4 and 16 h of LPS incubation. It was absent in LPS-free microglia, and after 24 h of LPS-treatment or later.(ABSTRACT TRUNCATED AT 250 WORDS)

Role for glial cells in regulating the functional expression of neuropeptide Y (NPY) neurons in aggregate cultures derived from dissociated fetal brain cells

A series of studies from our laboratory have established an aggregate culture system of fetal rat brain cells expressing neuropeptide Y (NPY) which can serve as a model to study the role of glia-neuron paracrine interactions in the developmental expression of NPY neurons. In this system, NPY production increases progressively with culture-age and it is induced by forskolin (FOR) and phorbol 12-myristate 13-acetate (PMA). We addressed the following question: Is the functional expression of the NPY neurons impaired in the absence of glial cells (particularly astrocytes) and if so, can secretory products of aggregates composed of the full complement of brain cells (intact aggregates) restore the function of the impaired NPY neurons? Aggregates were generated from 17-day-old fetal rat cortex and maintained in serum-free medium for 13-15 days. Cytosine arabinoside (CA; doses of 0.5-8 microM) was added to the cultures on day 1 and the effectiveness in elimination of glial cells was verified on day 15 by measuring the incorporation of 3H thymidine into DNA and by immunostaining for the astrocyte marker glial fibrillary acidic protein (GFAP). Basal NPY production and FOR (10 microM) + PMA (20 nM) stimulated production of NPY on days 13-15 were taken as functional criteria. FOR + PMA induced approximately 2-fold increase in NPY production in control cultures (no CA). CA inhibited both basal and FOR + PMA induced production of NPY and DNA synthesis in a dose-dependent manner: at 6 microM CA, basal NPY production was reduced by about 50%, FOR + PMA stimulated production of NPY and DNA synthesis were completely inhibited, and astrocytes were essentially eliminated.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes of beta-amyloid precursor protein splice patterns in brain cell aggregate cultures

The splice pattern of beta-amyloid precursor protein (beta-APP) has been studied in a variety of neuronal and glial cells and in brain cell aggregate cultures by the polymerase chain reaction (PCR). The brain-typical pattern, in which beta-APP695 is the dominant form, has been found only in aggregate cultures but not in any of the other cell types including neuronal cell lines. Selective elimination of glial cells from aggregates resulted in increased quantities of beta-APP695, whereas removal of neurons led to a reduction of beta-APP695 and to an elevation of beta-APP751 and beta-APP770. This shift of splice pattern was not observed in cocultures of the neuronal cell line PC 12 with primary astrocytes combined in a variety of cellular ratios. Blood serum, which is an essential component of these cultures, tested on aggregates, did not reduce the amount of beta-APP695 or have any marked effects on splice patterns generally. From these results it is concluded that investigations on brain-typical splicing of beta-APP require primary neurons. Neuronal cell lines may be no suitable model systems. Splicing events favoring production of beta-APP695 may mark an important, very early step of amyloid formation in the brain.

Glutamate up-regulates alpha 1 and alpha 2 subunits of the sodium pump in astrocytes of mixed telencephalic cultures but not in pure astrocyte cultures

Prior work employing an in vitro model of the cerebral cortex has shown that sodium pump activity is a critical determinant for neuronal survival of glutamate stimulation. We have hypothesized that up-regulation of total brain sodium pump activity will protect against potential excitotoxins. Increased sodium pump activity could theoretically occur by changes in the reaction rate (short-term) and/or by increased levels of sodium pump protein (long-term) and is potentially complex since the three catalytic (a) subunit isoforms of the sodium pump are distributed in a highly variable, cell-specific pattern in the brain. Short-term regulation (seconds to minutes) has been well studied: brain sodium pump exhibits a large dynamic range. In contrast, the possibility of long-term modulation of sodium pump activity has not been extensively explored. We used isoform specific antibodies and [3H]ouabain binding to determine whether prolonged stimulation of sodium pump activity in rodent telencephalic cultures increased total sodium pump enzyme. Exposure of mixed neuronal-glial cultures to high levels of glutamate (10 mM) for 18 h, which is highly toxic to neurons, was associated with an approximately 80% increase in alpha 1 and alpha 2 subunit expression by glia. Induction of alpha 2 subunit immunoreactivity was also associated with comparable changes in [3H]ouabain binding, suggesting that the up-regulation corresponded to functional alpha 2 protein. Shorter (30 min) glutamate treatments, which also killed neurons, did not produce similar changes in sodium pump expression. In contrast to mixed cultures, pure astrocyte cultures had undetectable alpha 2 and alpha 3 and moderate levels of alpha 1 protein, as confirmed by low levels of [3H]ouabain binding. Glutamate treatment using this protocol was associated with a decrease in alpha 1 sodium pump expression. We conclude that long-term regulation of the sodium pump can be demonstrated in glia which have developed in the presence of neurons. Both alpha 1 and alpha 2 isoforms of the sodium pump are involved in this response to glutamate.

Cell-type specific expression of Na+, K(+)-ATPase catalytic subunits in cultured neurons and glia: evidence for polarized distribution in neurons

Na+,K(+)-ATPase (the sodium pump) is a family of proteins consisting of catalytic (alpha) and glycoprotein (beta) subunit isoforms which are differentially expressed in excitable tissue. To gain insight into the cell-type distribution of sodium pump protein, we determined the expression pattern of fetal rat telencephalic cultures, of telencephalic cultures depleted of neurons, and of pure astrocyte cultures. Isoform-specific antibodies were used for immunoblotting and immunohistochemistry, with supplemental [3H]ouabain binding to assess levels of functional alpha 2/alpha 3 protein. The results show that neurons of mixed telencephalic cultures uniquely express alpha 3 and high levels of alpha 1. The marked similarity in the distribution of microtubule-associated protein-2 and alpha 1 immunocytochemical staining strongly suggests that alpha 1 subunits are enriched in dendrites. Further, highly correlative growth cone-associated protein-43 and alpha 3 staining is consistent with a preferential expression of alpha 3 subunits in axons, which are also characterized by low levels of alpha 1 and no alpha 2 immunoreactivity. Process-bearing glia are intimately associated with neuronal aggregates and express high levels of both alpha 1 and alpha 2 protein, as well as GFAP. Interestingly, polygonal, flat glia not within neuronal aggregates are weakly immunopositive only for alpha 1 and GFAP. Pure astrocytic cultures possess appreciable alpha 1 protein and GFAP, but lack both alpha 2 and alpha 3 immunoreactivity. As predicted by the immunohistochemical findings, [3H]ouabain binding was low in pure astrocytic cultures, and much higher in the neuron-enriched mixed cultures. These observations confirm that neurons express all three catalytic isoforms of the sodium pump. They also suggest that specific alpha-isoforms may be polarized to targeted membrane regions of neurons. Further, glia intimately associated with neurons express alpha 2, bind significant amounts of [3H]ouabain, and possess much higher levels of alpha 1 and GFAP compared to glia not near neurons. Thus, neurons may regulate glial sodium pump expression.

Quantification of rat cerebral cortex Na+,K(+)-ATPase: effect of age and potassium depletion

Na+,K(+)-ATPase concentration in rat cerebral cortex was studied by vanadate-facilitated [3H]ouabain binding to intact samples and by K(+)-dependent 3-O-methylfluorescein phosphatase activity determinations in crude homogenates. Methodological errors of both methods were evaluated. [3H]Ouabain binding to cerebral cortex obtained from 12-week-old rats measured incubating samples in buffer containing [3H]ouabain, and ouabain at a final concentration of 1 x 10(-6) mol/L gave a value of 11,351 +/- 177 (n = 5) pmol/g wet weight (mean +/- SEM) without any significant variation between the lobes. Evaluation of affinity for ouabain was in agreement with a heterogeneous population of [3H]ouabain binding sites. K(+)-dependent 3-O-methylfluorescein phosphatase activity in crude cerebral homogenates of age-matched rats was 7.24 +/- 0.14 (n = 5) mumol/min/g wet weight, corresponding to a Na+,K(+)-ATPase concentration of 12,209 +/- 236 pmol/g wet weight. It was concluded that the present methods were suitable for quantitative studies of cerebral cortex Na+,K(+)-ATPase. The concentration of rat cerebral cortex Na+,K(+)-ATPase showed approximately 10-fold increase within the first 4 weeks of life to reach a plateau of approximately 11,000-12,000 pmol/g wet weight, indicating a larger synthesis of Na+,K+ pumps than tissue mass in rat cerebral cortex during the first 4 weeks of development. K+ depletion induced by K(+)-deficient fodder for 2 weeks resulted in a slight tendency toward a reduction in K+ content (6%, p > 0.5) and Na+,K(+)-ATPase concentration (3%, p > 0.4) in cerebral cortex, whereas soleus muscle K+ content and Na+,K(+)-ATPase concentration were decreased by 30 (p < 0.02) and 32% (p < 0.001), respectively. Hence, during K+ depletion, cerebral cortex can maintain almost normal K+ homeostasis, whereas K+ as well as Na+,K+ pumps are lost from skeletal muscles.

Expression of the proto-oncogene c-fos in three-dimensional fetal brain cell cultures and the lack of correlation with maturation-inducing stimuli

Previous work has shown that aggregating fetal brain cell cultures are able to attain a highly differentiated state, and that their development is greatly enhanced by growth and/or differentiation factors such as epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), and the protein kinase C-activating tumor promoter mezerein. The present study shows that in these 3-dimensional cultures the peptide growth factors EGF and bFGF as well as mezerein are able to induce the expression of the proto-oncogene c-fos. This induction was rapid and transient, in good agreement with observations reported from a wide variety of cell types in vitro. The maximal levels of c-fos mRNA found after stimulation were low in immature cultures and increased greatly as maturation progressed. Of the three factors tested, mezerein was the most potent inducer of c-fos. In contrast to the peptide growth factors EGF and bFGF which were found to induce c-fos only in glial cells, mezerein was stimulatory in glial cells as well as in neurons. A similar cell type specificity has been observed previously for the maturation-enhancing response in immature aggregate cultures. However, in the present study no correlation was found between the degree of c-fos induction and the extent of the maturation-enhancing stimulation. Immature cultures known to be most sensitive and responsive to these maturation-enhancing agents required relatively high doses of peptide growth factors for the induction of c-fos, and the maximal levels of c-fos mRNA elicited were much lower than those in differentiated cultures which did not show any long-term response to these stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)