Cardiac Oxidative Signaling and Physiological Hypertrophy in the Na/K-ATPase α1s/sα2s/s Mouse Model of High Affinity for Cardiotonic Steroids

The Na/K-ATPase is the specific receptor for cardiotonic steroids (CTS) such as ouabain and digoxin. At pharmacological concentrations used in the treatment of cardiac conditions, CTS inhibit the ion-pumping function of Na/K-ATPase. At much lower concentrations, in the range of those reported for endogenous CTS in the blood, they stimulate hypertrophic growth of cultured cardiac myocytes through initiation of a Na/K-ATPase-mediated and reactive oxygen species (ROS)-dependent signaling. To examine a possible effect of endogenous concentrations of CTS on cardiac structure and function in vivo, we compared mice expressing the naturally resistant Na/K-ATPase α1 and age-matched mice genetically engineered to express a mutated Na/K-ATPase α1 with high affinity for CTS. In this model, total cardiac Na/K-ATPase activity, α1, α2, and β1 protein content remained unchanged, and the cardiac Na/K-ATPase dose–response curve to ouabain shifted to the left as expected. In males aged 3–6 months, increased α1 sensitivity to CTS resulted in a significant increase in cardiac carbonylated protein content, suggesting that ROS production was elevated. A moderate but significant increase of about 15% of the heart-weight-to-tibia-length ratio accompanied by an increase in the myocyte cross-sectional area was detected. Echocardiographic analyses did not reveal any change in cardiac function, and there was no fibrosis or re-expression of the fetal gene program. RNA sequencing analysis indicated that pathways related to energy metabolism were upregulated, while those related to extracellular matrix organization were downregulated. Consistent with a functional role of the latter, an angiotensin-II challenge that triggered fibrosis in the α1^r/rα2^s/s mouse failed to do so in the α1^s/sα2^s/s. Taken together, these results are indicative of a link between circulating CTS, Na/K-ATPase α1, ROS, and physiological cardiac hypertrophy in mice under baseline laboratory conditions.

KLF2 in Myeloid Lineage Cells Regulates the Innate Immune Response during Skeletal Muscle Injury and Regeneration

Skeletal muscle repair and regeneration after injury requires coordinated interactions between the innate immune system and the injured muscle. Myeloid cells predominate in these interactions. This study examined the role of KLF2, a zinc-finger transcription factor that regulates immune cell activation, in specifying myeloid cell functions during muscle regeneration. Loss of KLF2 in myeloid lineage cells (myeKlf2^-/- mice) dramatically enhanced the initial inflammatory response to acute muscle injury (cardiotoxin). Injured muscles showed dramatically elevated expression of inflammatory mediators and greater numbers of infiltrating, pro-inflammatory monocytes that matured earlier into activated macrophages. Notably, the inflammatory phase resolved earlier and regeneration progressed to myogenesis, marked by elevated expression of factors that promote the formation of new fibers from satellite cells. Regeneration was completed earlier, with phenotypically normal adult fibers integrated into the muscle syncytium. These findings identify myeloid KLF2 as a key regulator of myeloid cell functions in adult skeletal muscle regeneration.

Isoform-specific role of Na/K-ATPase α1 in skeletal muscle

The distribution of Na/K-ATPase α-isoforms in skeletal muscle is unique, with α1 as the minor (15%) isoform and α2 comprising the bulk of the Na/K-ATPase pool. The acute and isoform-specific role of α2 in muscle performance and resistance to fatigue is well known, but the isoform-specific role of α1 has not been as thoroughly investigated. In vitro, we reported that α1 has a role in promoting cell growth that is not supported by α2. To assess whether α1 serves this isoform-specific trophic role in the skeletal muscle, we used Na/K-ATPase α1-haploinsufficient (α1^+/-) mice. A 30% decrease of Na/K-ATPase α1 protein expression without change in α2 induced a modest yet significant decrease of 10% weight in the oxidative soleus muscle. In contrast, the mixed plantaris and glycolytic extensor digitorum longus weights were not significantly affected, likely because of their very low expression level of α1 compared with the soleus. The soleus mass reduction occurred without change in total Na/K-ATPase activity or glycogen metabolism. Serum analytes including K⁺, fat tissue mass, and exercise capacity were not altered in α1^+/- mice. The impact of α1 content on soleus muscle mass is consistent with a Na/K-ATPase α1-specific role in skeletal muscle growth that cannot be fulfilled by α2. The preserved running capacity in α1^+/- is in sharp contrast with previously reported consequences of genetic manipulation of α2. Taken together, these results lend further support to the concept of distinct isoform-specific functions of Na/K-ATPase α1 and α2 in skeletal muscle.

Abstract P331: Angiotensin-salt Hypertension Requires Ouabain-sensitive Na + Pumps

Dietary salt is a major factor in the pathogenesis of essential hypertension (EH), but the underlying links are unresolved. Animal models indicate that angiotensin (Ang) II and high dietary salt (HS) are convergent signals that act via the brain to elevate blood pressure (BP). Low-dose sc Ang II+HS is a common model for EH. We tested the Na + pump ouabain binding site’s role in this model because it is crucial in some other hypertension models (e.g., ACTH and Nedd4-2-knockout +HS). Mice that express Na + pumps with a mutant, ouabain-resistant α2 catalytic subunit (α2 R/R ; cation transport is normal), and wild type (WT), ouabain sensitive controls (α2 S/S ) were studied. [80-90% of rodent artery myocyte Na + pumps are ouabain-resistant (α1 R/R ); only 10-20% are α2.] BP was measured by telemetry. First, 3 basal 24 hr BPs were recorded. Osmotic 4-week minipumps were then implanted sc in all mice to deliver vehicle (saline; Expt. #1,3), or 400 (Expt. #1,2) or 800 (Expt. #3) ng/kg/min Ang II; simultaneously, in Expt. #2, the diet was switched from 0.4% (standard) to 2% NaCl (HS). BPs were monitored every 3-4 days for up to 4 weeks. Also, in Expt. #2, on day 21, all mice received 2 ip injections, 4 hrs apart, of 10 mg/kg DigiFab, Fab fragments that immuno-neutralize ouabain, while BP was continuously monitored; on day 23, the mice received 2 ip injections of CroFab, anti-crotalus toxin (‘control’) Fab fragments.

Results: 1. Basal mean BP (MBP) was 10±2 mm Hg higher in α2 R/R than in WT mice ( P <0.01; n =21 & 29; ANOVA). 2. In WT mice, 400 ng/kg/min sc Ang II and Ang II+HS raised MBP by 15±1 and 34±1 mm Hg, respectively ( P <0.01; n =7-8; ANOVA). 3. The MPB elevation in Ang II+HS α2 R/R (17±2 mm Hg) was only half that in WT mice ( P <0.01; n =7 each; ANOVA). 4. DigiFab rapidly (<1 hr) reduced MBP by 14±2 mm Hg in Ang II+HS hypertensive WT mice ( P <0.001; n =7; T-test), but not in α2 R/R mice ( P <0.01; n =7 each; ANOVA); CroFab did not lower MBP in either strain. 5. 800 ng/kg/min sc Ang II elevated systolic BP by 55±3 mm Hg in WT mice, but by only 37±3 mm Hg in α2 R/R mice ( P <0.05; n =3-5; ANOVA).

Conclusions: Ouabain-sensitive α2 Na + pumps and their endogenous ligand are both required for full expression of low-dose Ang II-salt hypertension. Ouabain-sensitive α2 pumps apparently also contribute to high-dose Ang II-hypertension.

Pivotal role of α2 Na+ pumps and their high affinity ouabain binding site in cardiovascular health and disease

Reduced smooth muscle (SM)-specific α2 Na⁺ pump expression elevates basal blood pressure (BP) and increases BP sensitivity to angiotensin II (Ang II) and dietary NaCl, whilst SM-α2 overexpression lowers basal BP and decreases Ang II/salt sensitivity. Prolonged ouabain infusion induces hypertension in rodents, and ouabain-resistant mutation of the α2 ouabain binding site (α2^R/R mice) confers resistance to several forms of hypertension. Pressure overload-induced heart hypertrophy and failure are attenuated in cardio-specific α2 knockout, cardio-specific α2 overexpression and α2^R/R mice. We propose a unifying hypothesis that reconciles these apparently disparate findings: brain mechanisms, activated by Ang II and high NaCl, regulate sympathetic drive and a novel neurohumoral pathway mediated by both brain and circulating endogenous ouabain (EO). Circulating EO modulates ouabain-sensitive α2 Na⁺ pump activity and Ca²⁺ transporter expression and, via Na⁺ /Ca²⁺ exchange, Ca²⁺ homeostasis. This regulates sensitivity to sympathetic activity, Ca²⁺ signalling and arterial and cardiac contraction.

Na+/Ca2+ exchange and Na+/K+-ATPase in the heart

This paper is the third in a series of reviews published in this issue resulting from the University of California Davis Cardiovascular Symposium 2014: Systems approach to understanding cardiac excitation–contraction coupling and arrhythmias: Na⁺ channel and Na⁺ transport. The goal of the symposium was to bring together experts in the field to discuss points of consensus and controversy on the topic of sodium in the heart. The present review focuses on cardiac Na⁺/Ca²⁺ exchange (NCX) and Na⁺/K⁺-ATPase (NKA). While the relevance of Ca²⁺ homeostasis in cardiac function has been extensively investigated, the role of Na⁺ regulation in shaping heart function is often overlooked. Small changes in the cytoplasmic Na⁺ content have multiple effects on the heart by influencing intracellular Ca²⁺ and pH levels thereby modulating heart contractility. Therefore it is essential for heart cells to maintain Na⁺ homeostasis. Among the proteins that accomplish this task are the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺/K⁺ pump (NKA). By transporting three Na⁺ ions into the cytoplasm in exchange for one Ca²⁺ moved out, NCX is one of the main Na⁺ influx mechanisms in cardiomyocytes. Acting in the opposite direction, NKA moves Na⁺ ions from the cytoplasm to the extracellular space against their gradient by utilizing the energy released from ATP hydrolysis. A fine balance between these two processes controls the net amount of intracellular Na⁺ and aberrations in either of these two systems can have a large impact on cardiac contractility. Due to the relevant role of these two proteins in Na⁺ homeostasis, the emphasis of this review is on recent developments regarding the cardiac Na⁺/Ca²⁺ exchanger (NCX1) and Na⁺/K⁺ pump and the controversies that still persist in the field.

Phospholemman is not required for the acute stimulation of Na⁺-K⁺-ATPase α₂-activity during skeletal muscle fatigue

The Na(+)-K(+)-ATPase α2-isoform in skeletal muscle is rapidly stimulated during muscle use and plays a critical role in fatigue resistance. The acute mechanisms that stimulate α2-activity are not completely known. This study examines whether phosphorylation of phospholemman (PLM/FXYD1), a regulatory subunit of Na(+)-K(+)-ATPase, plays a role in the acute stimulation of α2 in working muscles. Mice lacking PLM (PLM KO) have a normal content of the α2-subunit and show normal exercise capacity, in contrast to the greatly reduced exercise capacity of mice that lack α2 in the skeletal muscles. Nerve-evoked contractions in vivo did not induce a change in total PLM or PLM phosphorylated at Ser63 or Ser68, in either WT or PLM KO. Isolated muscles of PLM KO mice maintain contraction and resist fatigue as well as wild type (WT). Rb(+) transport by the α2-Na(+)-K(+)-ATPase is stimulated to the same extent in contracting WT and contracting PLM KO muscles. Phosphorylation of sarcolemmal membranes prepared from WT but not PLM KO skeletal muscles stimulates the activity of both α1 and α2 in a PLM-dependent manner. The stimulation occurs by an increase in Na(+) affinity without significant change in Vmax and is more effective for α1 than α2. These results demonstrate that phosphorylation of PLM is capable of stimulating the activity of both isozymes in skeletal muscle; however, contractile activity alone is not sufficient to induce PLM phosphorylation. Importantly, acute stimulation of α2, sufficient to support exercise and oppose fatigue, does not require PLM or its phosphorylation.

Na,K-ATPase α2 activity in mammalian skeletal muscle T-tubules is acutely stimulated by extracellular K+

The K⁺ affinity of the Na,K-ATPase α2 isoform matches its activity to the range of extracellular K⁺ concentrations in the T-tubules at rest and during contraction, maintaining the excitability of active muscle.

The Na,K-ATPase α2 isoform is the predominant Na,K-ATPase in adult skeletal muscle and the sole Na,K-ATPase in the transverse tubules (T-tubules). In quiescent muscles, the α2 isozyme operates substantially below its maximal transport capacity. Unlike the α1 isoform, the α2 isoform is not required for maintaining resting ion gradients or the resting membrane potential, canonical roles of the Na,K-ATPase in most other cells. However, α2 activity is stimulated immediately upon the start of contraction and, in working muscles, its contribution is crucial to maintaining excitation and resisting fatigue. Here, we show that α2 activity is determined in part by the K⁺ concentration in the T-tubules, through its K⁺ substrate affinity. Apparent K⁺ affinity was determined from measurements of the K_1/2 for K⁺ activation of pump current in intact, voltage-clamped mouse flexor digitorum brevis muscle fibers. Pump current generated by the α2 Na,K-ATPase, Ip, was identified as the outward current activated by K⁺ and inhibited by micromolar ouabain. Ip was outward at all potentials studied (−90 to −30 mV) and increased with depolarization in the subthreshold range, −90 to −50 mV. The Q₁₀ was 2.1 over the range of 22–37°C. The K_1/2,K of Ip was 4.3 ± 0.3 mM at −90 mV and was relatively voltage independent. This K⁺ affinity is lower than that reported for other cell types but closely matches the dynamic range of extracellular K⁺ concentrations in the T-tubules. During muscle contraction, T-tubule luminal K⁺ increases in proportion to the frequency and duration of action potential firing. This K_1/2,K predicts a low fractional occupancy of K⁺ substrate sites at the resting extracellular K⁺ concentration, with occupancy increasing in proportion to the frequency of membrane excitation. The stimulation of preexisting pumps by greater K⁺ site occupancy thus provides a rapid mechanism for increasing α2 activity in working muscles.

An α2-Na/K ATPase/α-adducin complex in astrocytes triggers non-cell autonomous neurodegeneration

Perturbations of astrocytes trigger neurodegeneration in several diseases, but the glial cell-intrinsic mechanisms that induce neurodegeneration remain poorly understood. We found that a protein complex of α2-Na/K ATPase and α-adducin was enriched in astrocytes expressing mutant superoxide dismutase 1 (SOD1), which causes familial amyotrophic lateral sclerosis (ALS). Knockdown of α2-Na/K ATPase or α-adducin in mutant SOD1 astrocytes protected motor neurons from degeneration, including in mutant SOD1 mice in vivo. Heterozygous disruption of the α2-Na/K ATPase gene suppressed degeneration in vivo and increased the lifespan of mutant SOD1 mice. The pharmacological agent digoxin, which inhibits Na/K ATPase activity, protected motor neurons from mutant SOD1 astrocyte-induced degeneration. Notably, α2-Na/K ATPase and α-adducin were upregulated in spinal cord of sporadic and familial ALS patients. Collectively, our findings define chronic activation of the α2-Na/K ATPase/α-adducin complex as a critical glial cell-intrinsic mechanism of non-cell autonomous neurodegeneration, with implications for potential therapies for neurodegenerative diseases.

Reduced levels of microRNAs miR-124a and miR-150 are associated with increased proinflammatory mediator expression in Krüppel-like factor 2 (KLF2)-deficient macrophages

Previous studies have shown that the myeloid-specific deficiency of the transcription factor Krüppel-like factor 2 (KLF2) accelerates atherosclerosis in hypercholesterolemic Ldlr(-/-) mice due to the enhanced adhesion of myeloid cells to activated endothelial cells in the vessel wall. This study revealed elevated basal inflammation with elevated plasma levels of Ccl2, Ccl4, Ccl5, and Ccl11 in the myeloid-specific KLF2 knock-out (myeKlf2(-/-)) mice. Peritoneal macrophages isolated from myeKlf2(-/-) mice showed increased mRNA levels of several inflammatory mediators, including Ccl2, Ccl5, Ccl7, Cox-2, Cxcl1, and IL-6. In contrast, the levels of two microRNAs, miR-124a and miR-150, were lower in Klf2(-/-) macrophages compared with Klf2(+/+) macrophages. Additional studies showed a direct inverse relationship between miR-124a levels with Ccl2 expression, with anti-miR-124a increasing Ccl2 mRNA levels in Klf2(+/+) macrophages, whereas the restoration of miR-124a levels in Klf2(-/-) macrophages significantly reduced Ccl2 mRNA expression. Likewise, the inverse relationship was observed between miR-150 levels and Cxcl1 expression in Klf2(+/+) and Klf2(-/-) mice. Moreover, miR150 likely regulates the miR124a expression and thus augments expression of inflammatory mediators in myeKlf2(-/-) macrophages. This study documented that the transcription factor KLF2 modulates inflammatory chemokine production via regulation of microRNA expression levels in immune cells.

Knockout of the Na,K-ATPase α2-isoform in cardiac myocytes delays pressure overload-induced cardiac dysfunction

The α2-isoform of the Na,K-ATPase (α2) is the minor isoform of the Na,K-ATPase expressed in the cardiovascular system and is thought to play a critical role in the regulation of cardiovascular hemodynamics. However, the organ system/cell type expressing α2 that is required for this regulation has not been fully defined. The present study uses a heart-specific knockout of α2 to further define the tissue-specific role of α2 in the regulation of cardiovascular hemodynamics. To accomplish this, we developed a mouse model using the Cre/loxP system to generate a tissue-specific knockout of α2 in the heart using β-myosin heavy chain Cre. We have achieved a 90% knockout of α2 expression in the heart of the knockout mice. Interestingly, the heart-specific knockout mice exhibit normal basal cardiac function and systolic blood pressure, and in addition, these mice develop ACTH-induced hypertension in response to ACTH treatment similar to control mice. Surprisingly, the heart-specific knockout mice display delayed onset of cardiac dysfunction compared with control mice in response to pressure overload induced by transverse aortic constriction; however, the heart-specific knockout mice deteriorated to control levels by 9 wk post-transverse aortic constriction. These results suggest that heart expression of α2 does not play a role in the regulation of basal cardiovascular function or blood pressure; however, heart expression of α2 plays a role in the hypertrophic response to pressure overload. This study further emphasizes that the tissue localization of α2 determines its unique roles in the regulation of cardiovascular function.

Tissue-specific role of the Na,K-ATPase α2 isozyme in skeletal muscle

The Na,K-ATPase α2 isozyme is the major Na,K-ATPase of mammalian skeletal muscle. This distribution is unique compared with most other cells, which express mainly the Na,K-ATPase α1 isoform, but its functional significance is not known. We developed a gene-targeted mouse (skα2(-/-)) in which the α2 gene (Atp1a2) is knocked out in the skeletal muscles, and examined the consequences for exercise performance, membrane potentials, contractility, and muscle fatigue. Targeted knockout was confirmed by genotyping, Western blot, and immunohistochemistry. Skeletal muscle cells of skα2(-/-) mice completely lack α2 protein and have no α2 in the transverse tubules, where its expression is normally enhanced. The α1 isoform, which is normally enhanced on the outer sarcolemma, is up-regulated 2.5-fold without change in subcellular targeting. skα2(-/-) mice are apparently normal under basal conditions but show significantly reduced exercise capacity when challenged to run. Their skeletal muscles produce less force, are unable to increase force to match demand, and show significantly increased susceptibility to fatigue. The impairments affect both fast and slow muscle types. The subcellular targeting of α2 to the transverse tubules is important for this role. Increasing Na,K-ATPase α1 content cannot fully compensate for the loss of α2. The increased fatigability of skα2(-/-) muscles is reproduced in control extensor digitorum longus muscles by selectively inhibiting α2 enzyme activity with ouabain. These results demonstrate that the Na,K-ATPase α2 isoform performs an acute, isoform-specific role in skeletal muscle. Its activity is regulated by muscle use and enables working muscles to maintain contraction and resist fatigue.

Abstract 416: Chloride Ion Contributes to the Hypertension Induced by Central NaCl

Previous studies in salt-sensitive animal strains suggest that an increase in the level of sodium in the cerebrospinal fluid (CSF [Na]) is crucial for the development of hypertension induced by a high salt diet. Although chronic intracerebroventricular (icv) infusions of NaCl-rich artificial CSF (NaCl-rich aCSF) are frequently used to study the mechanisms of the hypertension caused by high salt diet and/or elevated CSF [Na], the contribution of chronic increased CSF [Cl] to the hypertensive response has not been examined. In the present study, we compared the effects on blood pressure of chronic (10 day) icv infusions of NaCl-rich aCSF vs. similar infusions in which excess Na+ or Cl- was substituted with another ion in wild-type (WT) mice and in knockout/knockin mice with a ouabain-resistant endogenous Na, K-ATPase α2 subunit (α2 R/R ). The latter mice are known to lack a pressor response to icv NaCl-rich aCSF. Mean arterial pressure (MAP) and heart rate (HR) on day 10 of infusion were (mean ± S.E.M., n = 4-5 per group:

These studies suggest that both elevated CSF [Na] and CSF [Cl] contribute to the pressor response to increased CSF [NaCl] via a common mechanism: binding of endogenous ouabain to the Na, K-ATPase α2 isoform.

Na(+)/K)+)-ATPase α2-isoform preferentially modulates Ca2(+) transients and sarcoplasmic reticulum Ca2(+) release in cardiac myocytes

AIMS

Na(+)/K(+)-ATPase (NKA) is essential in regulating [Na(+)](i), and thus cardiac myocyte Ca(2+) and contractility via Na(+)/Ca(2+) exchange. Different NKA-α subunit isoforms are present in the heart and may differ functionally, depending on specific membrane localization. In smooth muscle and astrocytes, NKA-α2 is located at the junctions with the endo(sarco)plasmic reticulum, where they could regulate local [Na(+)], and indirectly junctional cleft [Ca(2+)]. Whether this model holds for cardiac myocytes is unclear.

METHODS AND RESULTS

The ouabain-resistant NKA-α1 cannot be selectively blocked to assess its effect. To overcome this, we used mice in which NKA-α1 is ouabain sensitive and NKA-α2 is ouabain resistant (SWAP mice). We measured the effect of ouabain at low concentration on [Na(+)](i), Ca(2+) transients, and the fractional sarcoplasmic reticulum (SR) Ca(2+) release in cardiac myocytes from wild-type (WT; NKA-α2 inhibition) and SWAP mice (selective NKA-α1 block). At baseline, Na(+) and Ca(2+) regulations are similar in WT and SWAP mice. For equal levels of total NKA inhibition (~25%), ouabain significantly increased Ca(2+) transients (from ΔF/F(0)= 1.5 ± 0.1 to 1.8 ± 0.1), and fractional SR Ca(2+) release (from 24 ± 3 to 29 ± 3%) in WT (NKA-α2 block) but not in SWAP myocytes (NKA-α1 block). This occurred despite a similar and modest increase in [Na(+)](i) (~2 mM) in both groups. The effect in WT mice was mediated specifically by NKA-α2 inhibition because at a similar concentration ouabain had no effect in transgenic mice where both NKA-α1 and NKA-α2 are ouabain resistant.

CONCLUSION

NKA-α2 has a more prominent role (vs. NKA-α1) in modulating cardiac myocyte SR Ca(2+) release.

Myeloid-specific Krüppel-like factor 2 inactivation increases macrophage and neutrophil adhesion and promotes atherosclerosis

RATIONALE

Hemizygous deficiency of the transcription factor Krüppel-like factor 2 (KLF2) has been shown previously to augment atherosclerosis in hypercholesterolemic mice. However, the cell type responsible for the increased atherosclerosis due to KLF2 deficiency has not been identified. This study examined the consequence of myeloid cell-specific KLF2 inactivation in atherosclerosis.

METHODS AND RESULTS

Cell-specific knockout mice were generated by Cre/loxP recombination. Macrophages isolated from myeloid-specific Klf2 knockout (myeKlf2(-/-)) mice were similar to myeKlf2(+/+) macrophages in response to activation, polarization, and lipid accumulation. However, in comparison to myeKlf2(+/+) macrophages, myeKlf2(-/-) macrophages adhered more robustly to endothelial cells. Neutrophils from myeKlf2(-/-) mice also adhered more robustly to endothelial cells, and fewer myeKlf2(-/-) neutrophils survived in culture over a 24-hour period in comparison with myeKlf2(+/+) neutrophils. When myeKlf2(-/-) mice were mated to Ldlr(-/-) mice and then fed a high fat and high cholesterol diet, significant increase in atherosclerosis was observed in the myeKlf2(-/-)Ldlr(-/-) mice compared with myeKlf2(+/+)Ldlr(-/-) littermates. The increased atherosclerosis in myeKlf2(-/-)Ldlr(-/-) mice was associated with elevated presence of neutrophils and macrophages, with corresponding increase of myeloperoxidase as well as chlorinated and nitrosylated tyrosine epitopes in their lesion areas compared with myeKlf2(+/+)Ldlr(-/-) mice.

CONCLUSIONS

This study documents a role for myeloid KLF2 expression in modulating atherosclerosis. The increased neutrophil accumulation and atherosclerosis progression with myeloid-specific KLF2 deficiency also underscores the importance of neutrophils in promoting vascular oxidative stress and atherosclerosis. Collectively, these results suggest that elevating KLF2 expression may be a novel strategy for prevention and treatment of atherosclerosis.

Reduction of Na/K-ATPase potentiates marinobufagenin-induced cardiac dysfunction and myocyte apoptosis

Decreases in cardiac Na/K-ATPase have been documented in patients with heart failure. Reduction of Na/K-ATPase α1 also contributes to the deficiency in cardiac contractility in animal models. Our previous studies demonstrate that reduction of cellular Na/K-ATPase causes cell growth inhibition and cell death in renal proximal tubule cells. To test whether reduction of Na/K-ATPase in combination with increased cardiotonic steroids causes cardiac myocyte death and cardiac dysfunction, we examined heart function in Na/K-ATPase α1 heterozygote knock-out mice (α1(+/-)) in comparison to wild type (WT) littermates after infusion of marinobufagenin (MBG). Adult cardiac myocytes were also isolated from both WT and α1(+/-) mice for in vitro experiments. The results demonstrated that MBG infusion increased myocyte apoptosis and induced significant left ventricle dilation in α1(+/-) mice but not in their WT littermates. Mechanistically, it was found that in WT myocytes MBG activated the Src/Akt/mTOR signaling pathway, which further increased phosphorylation of ribosome S6 kinase (S6K) and BAD (Bcl-2-associated death promoter) and protected cells from apoptosis. In α1(+/-) myocytes, the basal level of phospho-BAD is higher compared with WT myocytes, but MBG failed to induce further activation of the mTOR pathway. Reduction of Na/K-ATPase also caused the activation of caspase 9 but not caspase 8 in these cells. Using cultures of neonatal cardiac myocytes, we demonstrated that inhibition of the mTOR pathway by rapamycin also enabled MBG to activate caspase 9 and induce myocyte apoptosis.

A myeloid hypoxia-inducible factor 1α-Krüppel-like factor 2 pathway regulates gram-positive endotoxin-mediated sepsis

Although gram-positive infections account for the majority of cases of sepsis, the molecular mechanisms underlying their effects remains poorly understood. We investigated how cell wall components of gram-positive bacteria contribute to the development of sepsis. Experimental observations derived from cultured primary macrophages and the cell line indicate that gram-positive bacterial endotoxins induce hypoxia-inducible factor 1α (HIF-1α) mRNA and protein expression. Inoculation of live or heat-inactivated gram-positive bacteria with macrophages induced HIF-1 transcriptional activity in macrophages. Concordant with these results, myeloid deficiency of HIF-1α attenuated gram-positive bacterial endotoxin-induced cellular motility and proinflammatory gene expression in macrophages. Conversely, gram-positive bacteria and their endotoxins reduced expression of the myeloid anti-inflammatory transcription factor Krüppel-like transcription factor 2 (KLF2). Sustained expression of KLF2 reduced and deficiency of KLF2 enhanced gram-positive endotoxins induced HIF-1α mRNA and protein expression in macrophages. More importantly, KLF2 attenuated gram-positive endotoxins induced cellular motility and proinflammatory gene expression in myeloid cells. Consistent with these results, mice deficient in myeloid HIF-1α were protected from gram-positive endotoxin-induced sepsis mortality and clinical symptomatology. By contrast, myeloid KLF2-deficient mice were susceptible to gram-positive sepsis induced mortality and clinical symptoms. Collectively, these observations identify HIF-1α and KLF2 as critical regulators of gram-positive endotoxin-mediated sepsis.

Hypertension from chronic central sodium chloride in mice is mediated by the ouabain-binding site on the Na,K-ATPase α₂-isoform

A chronic increase in the concentration of sodium chloride in the cerebrospinal fluid (CSF) (↑CSF [NaCl]) appears to be critically important for the development of salt-dependent hypertension. In agreement with this concept, increasing CSF [NaCl] chronically by intracerebroventricular (icv) infusion of NaCl-rich artificial CSF (aCSF-HiNaCl) in rats produces hypertension by the same mechanisms (i.e., aldosterone-ouabain pathway in the brain) as that produced by dietary sodium in salt-sensitive strains. We first demonstrate here that icv aCSF-HiNaCl for 10 days also causes hypertension in wild-type (WT) mice. We then used both WT and gene-targeted mice to explore the mechanisms. In WT mice with a ouabain-sensitive Na,K-ATPase α(2)-isoform (α2(S/S)), mean arterial pressure rose by ~25 mmHg within 2 days of starting aCSF-HiNaCl (0.6 nmol Na/min) and remained elevated throughout the study. Ouabain (171 pmol/day icv) increased blood pressure to a similar extent. aCSF-HiNaCl or ouabain given at the same rates subcutaneously instead of intracerebroventricularly had no effect on blood pressure. The pressor response to icv aCSF-HiNaCl was abolished by an anti-ouabain antibody given intracerebroventricularly but not subcutaneously, indicating that it is mediated by an endogenous ouabain-like substance in the brain. We compared the effects of icv aCSF-HiNaCl or icv ouabain on blood pressure in α2(S/S) versus knockout/knockin mice with a ouabain-resistant endogenous α(2)-subunit (α2(R/R)). In α2(R/R), there was no pressor response to icv aCSF-HiNaCl in contrast to WT mice. The α2(R/R) genotype also lacked a pressor response to icv ouabain. These data demonstrate that chronic ↑CSF [NaCl] causes hypertension in mice and that the blood pressure response is mediated by the ouabain-like substance in the brain, specifically by its binding to the α(2)-isoform of the Na,K-ATPase.

Knockout of the Na,K-ATPase α₂-isoform in the cardiovascular system does not alter basal blood pressure but prevents ACTH-induced hypertension

The α(2)-isoform of Na,K-ATPase (α(2)) is thought to play a role in blood pressure regulation, but the specific cell type(s) involved have not been identified. Therefore, it is important to study the role of the α(2) in individual cell types in the cardiovascular system. The present study demonstrates the role of vascular smooth muscle α(2) in the regulation of cardiovascular hemodynamics. To accomplish this, we developed a mouse model utilizing the Cre/LoxP system to generate a cell type-specific knockout of the α(2) in vascular smooth muscle cells using the SM22α Cre. We achieved a 90% reduction in the α(2)-expression in heart and vascular smooth muscle in the knockout mice. Interestingly, tail-cuff blood pressure analysis reveals that basal systolic blood pressure is unaffected by the knockout of α(2) in the knockout mice. However, knockout mice do fail to develop ACTH-induced hypertension, as seen in wild-type mice, following 5 days of treatment with ACTH (Cortrosyn; wild type = 119.0 ± 6.8 mmHg; knockout = 103.0 ± 2.0 mmHg). These results demonstrate that α(2)-expression in heart and vascular smooth muscle is not essential for regulation of basal systolic blood pressure, but α(2) is critical for blood pressure regulation under chronic stress such as ACTH-induced hypertension.

Histone deacetylase 9 is a negative regulator of adipogenic differentiation

Differentiation of preadipocytes into mature adipocytes capable of efficiently storing lipids is an important regulatory mechanism in obesity. Here, we examined the involvement of histone deacetylases (HDACs) and histone acetyltransferases (HATs) in the regulation of adipogenesis. We find that among the various members of the HDAC and HAT families, only HDAC9 exhibited dramatic down-regulation preceding adipogenic differentiation. Preadipocytes from HDAC9 gene knock-out mice exhibited accelerated adipogenic differentiation, whereas HDAC9 overexpression in 3T3-L1 preadipocytes suppressed adipogenic differentiation, demonstrating its direct role as a negative regulator of adipogenesis. HDAC9 expression was higher in visceral as compared with subcutaneous preadipocytes, negatively correlating with their potential to undergo adipogenic differentiation in vitro. HDAC9 localized in the nucleus, and its negative regulation of adipogenesis segregates with the N-terminal nuclear targeting domain, whereas the C-terminal deacetylase domain is dispensable for this function. HDAC9 co-precipitates with USF1 and is recruited with USF1 at the E-box region of the C/EBPα gene promoter in preadipocytes. Upon induction of adipogenic differentiation, HDAC9 is down-regulated, leading to its dissociation from the USF1 complex, whereas p300 HAT is up-regulated to allow its association with USF1 and accumulation at the E-box site of the C/EBPα promoter in differentiated adipocytes. This reciprocal regulation of HDAC9 and p300 HAT in the USF1 complex is associated with increased C/EBPα expression, a master regulator of adipogenic differentiation. These findings provide new insights into mechanisms of adipogenic differentiation and document a critical regulatory role for HDAC9 in adipogenic differentiation through a deacetylase-independent mechanism.

Renovascular hypertension using a modified two-kidney, one-clip approach in mice is not dependent on the α1 or α2 Na-K-ATPase ouabain-binding site

Endogenous cardiotonic steroids, through their interaction with the ouabain-binding site of the Na-K-ATPase α-subunit, have been implicated in a variety of cardiovascular disease states including hypertension. We have previously shown that ACTH-induced hypertension is abolished in mutant mice expressing ouabain-resistant α1- and α2-subunits. To further evaluate hypertension resistance in these mutant mice, we examined blood pressure changes in a modified model of 2-kidney, 1-clip (2K1C) renovascular hypertension. To reliably generate 2K1C hypertension, we used polyvinyl tubing (inner diameter: ∼0.27 mm) to accurately gauge the degree of renal artery stenosis. Using this method, virtually all of the clipped mice became hypertensive and there was no incidence of apparent renal ischemia. By telemetry, in response to renal artery clipping, blood pressure in wild-type mice (α1 ouabain-resistant, α2 ouabain-sensitive) increased from 97 ± 3 to 136 ± 7 mmHg. In α1-resistant, α2-resistant mice, pressure increased from 93 ± 2 to 123 ± 4 mmHg, and in α1-sensitive, α2-resistant mice, blood pressure increased from 95 ± 2 to 139 ± 5 mmHg. Blood pressure changes were equivalent in all three groups. In sham mice, blood pressure did not change (96 ± 1 to 95 ± 2 mmHg). Renin mRNA expression was dramatically elevated in the left vs. the right kidney, and plasma renin concentration was elevated similarly in all genotypes. These data indicate that sensitivity of the α1- or α2-Na-K-ATPase binding site to cardiotonic steroids is not a prerequisite for the development of 2K1C renovascular hypertension. In addition, use of a polyurethane cuff to constrict the renal artery provides a reliable method for producing 2K1C hypertension in mice.

The myeloid transcription factor KLF2 regulates the host response to polymicrobial infection and endotoxic shock

Precise control of myeloid cell activation is required for optimal host defense. However, this activation process must be under exquisite control to prevent uncontrolled inflammation. Herein, we identify the Kruppel-like transcription factor 2 (KLF2) as a potent regulator of myeloid cell activation in vivo. Exposure of myeloid cells to hypoxia and/or bacterial products reduced KLF2 expression while inducing hypoxia inducible factor-1α (HIF-1α), findings that were recapitulated in human septic patients. Myeloid KLF2 was found to be a potent inhibitor of nuclear factor-kappaB (NF-κB)-dependent HIF-1α transcription and, consequently, a critical determinant of outcome in models of polymicrobial infection and endotoxemia. Collectively, these observations identify KLF2 as a tonic repressor of myeloid cell activation in vivo and an essential regulator of the innate immune system.

Targeted mutations in the Na,K-ATPase α 2 isoform confer ouabain resistance and result in abnormal behavior in mice

Sodium and potassium-activated adenosine triphosphatases (Na,K-ATPase) are ubiquitous, participate in osmotic balance and membrane potential, and are composed of α, β, and γ subunits. The α subunit is required for the catalytic and transport properties of the enzyme and contains binding sites for cations, ATP, and digitalis-like compounds including ouabain. There are four known α isoforms; three that are expressed in the CNS in a regional and cell-specific manner. The α2 isoform is most commonly found in astrocytes, pyramidal cells of the hippocampus in adults, and developmentally in several other neuronal types. Ouabain-like compounds are thought to be produced endogenously in mammals, bind the Na,K-ATPase, and function as a stress-related hormone, however, the impact of the Na,K-ATPase ouabain binding site on neurobehavioral function is largely unknown. To determine if the ouabain binding site of the α2 isoform plays a physiological role in CNS function, we examined knock-in mice in which the normally ouabain-sensitive α2 isoform was made resistant (α2(R/R) ) while still retaining basal Na,K-ATPase enzymatic function. Egocentric learning (Cincinnati water maze) was impaired in adult α2(R/R) mice compared to wild type (WT) mice. They also exhibited decreased locomotor activity in a novel environment and increased responsiveness to a challenge with an indirect sympathomimetic agonist (methamphetamine) relative to WT mice. The α2(R/R) mice also demonstrated a blunted acoustic startle reflex and a failure to habituate to repeated acoustic stimuli. The α2(R/R) mice showed no evidence of altered anxiety (elevated zero maze) nor were they impaired in spatial learning or memory in the Morris water maze and neither group could learn in a large Morris maze. These results suggest that the ouabain binding site is involved in specific types of learning and the modulation of dopamine-mediated locomotor behavior.

Mice expressing ouabain-sensitive α1-Na,K-ATPase have increased susceptibility to pressure overload-induced cardiac hypertrophy

The Na,K-ATPase is a ubiquitous transmembrane pump and a specific receptor for cardiac glycosides such as ouabain and digoxin, which are used in the management of congestive heart failure (CHF). A potential role for these so-called endogenous cardiotonic steroids (CS) has been explored, and it has become apparent that such compounds are elevated and may play an important role in a variety of physiological and pathophysiological conditions such as hypertension and CHF. Recent evidence suggests that the Na,K-ATPase may act as a signal transducer upon CS binding and induce nonproliferative cardiac growth, implicating a role for endogenous CS in the development of cardiac hypertrophy and progressive failure of the heart. In the present study, we tested whether hypertrophic responses to pressure overload would be altered in mutant mice that specifically express ouabain-sensitive or ouabain-resistant α1- and α2-Na,K-ATPase subunits, as follows: α1-resistant, α2-resistant (α1(R/R)α2(R/R)); α1-sensitive, α2-resistant (α1(S/S)α2(R/R)); and α1-resistant, α2-sensitive (α1(R/R)α2(S/S), wild-type). In α1(S/S)α2(R/R) mice, pressure overload by transverse aortic coarctation induced severe left ventricular (LV) hypertrophy with extensive perivascular and replacement fibrosis at only 4 wk. Responses in α1(R/R)α2(S/S) and α1(R/R)α2(R/R) mice were comparatively mild. Mutant α1(S/S)α2(R/R) mice also had LV dilatation and depressed LV systolic contractile function by 4 wk of pressure overload. In separate experiments, chronic Digibind treatment prevented the rapid progression of cardiac hypertrophy and fibrosis in α1(S/S)α2(R/R) mice. These data demonstrate that mice with a ouabain-sensitive α1-Na,K-ATPase subunit have a dramatic susceptibility to the development of cardiac hypertrophy, and failure from LV pressure overload and provide evidence for the involvement of endogenous CS in this process.

The physiological significance of the cardiotonic steroid/ouabain-binding site of the Na,K-ATPase

The Na,K-ATPase is the membrane "pump" that generates the Na(+) and K(+) gradients across the plasma membrane that drives many physiological processes. This enzyme is highly sensitive to inhibition by cardiotonic steroids, most notably the digitalis/ouabain class of compounds, which have been used for centuries to treat congestive heart failure and arrhythmias. The amino acids that constitute the ouabain-binding site are highly conserved across the evolutionary spectrum. This could be fortuitous or could result from this site being conserved because it has an important biological function. New physiological approaches using genetically engineered mice are being used to define the biological significance of the "receptor function" of the Na,K-ATPase and its regulation by potential endogenous cardiotonic steroid-like compounds. These studies extend the reach of earlier studies involving the biochemical purification of endogenous regulatory ligands.

Isoform specificity of the Na/K-ATPase association and regulation by phospholemman

Phospholemman (PLM) phosphorylation mediates enhanced Na/K-ATPase (NKA) function during adrenergic stimulation of the heart. Multiple NKA isoforms exist, and their function/regulation may differ. We combined fluorescence resonance energy transfer (FRET) and functional measurements to investigate isoform specificity of the NKA-PLM interaction. FRET was measured as the increase in the donor fluorescence (CFP-NKA-alpha1 or CFP-NKA-alpha2) during progressive acceptor (PLM-YFP) photobleach in HEK-293 cells. Both pairs exhibited robust FRET (maximum of 23.6 +/- 3.4% for NKA-alpha1 and 27.5 +/- 2.5% for NKA-alpha2). Donor fluorescence depended linearly on acceptor fluorescence, indicating a 1:1 PLM:NKA stoichiometry for both isoforms. PLM phosphorylation induced by cAMP-dependent protein kinase and protein kinase C activation drastically reduced the FRET with both NKA isoforms. However, submaximal cAMP-dependent protein kinase activation had less effect on PLM-NKA-alpha2 versus PLM-NKA-alpha1. Surprisingly, ouabain virtually abolished NKA-PLM FRET but only partially reduced co-immunoprecipitation. PLM-CFP also showed FRET to PLM-YFP, but the relationship during progressive photobleach was highly nonlinear, indicating oligomers involving >or=3 monomers. Using cardiac myocytes from wild-type mice and mice where NKA-alpha1 is ouabain-sensitive and NKA-alpha2 is ouabain-resistant, we assessed the effects of PLM phosphorylation on NKA-alpha1 and NKA-alpha2 function. Isoproterenol enhanced internal Na(+) affinity of both isoforms (K((1/2)) decreased from 18.1 +/- 2.0 to 11.5 +/- 1.9 mm for NKA-alpha1 and from 16.4 +/- 2.5 to 10.4 +/- 1.5 mm for NKA-alpha2) without altering maximum transport rate (V(max)). Protein kinase C activation also decreased K((1/2)) for both NKA-alpha1 and NKA-alpha2 (to 9.4 +/- 1.0 and 9.1 +/- 1.1 mm, respectively) but increased V(max) only for NKA-alpha2 (1.9 +/- 0.4 versus 1.2 +/- 0.5 mm/min). In conclusion, PLM associates with and modulates both NKA-alpha1 and NKA-alpha2 in a comparable but not identical manner.

Extracellular potassium dependence of the Na+-K+-ATPase in cardiac myocytes: isoform specificity and effect of phospholemman

Cardiac Na(+)-K(+)-ATPase (NKA) regulates intracellular Na(+), which in turn affects intracellular Ca(2+) and contractility via the Na(+)/Ca(2+) exchanger. Extracellular K(+) concentration ([K(+)]) is a central regulator of NKA activity. Phospholemman (PLM) has recently been recognized as a critical regulator of NKA in the heart. PLM reduces the intracellular Na(+) affinity of NKA, an effect relieved by PLM phosphorylation. Here we tested whether the NKA alpha(1)- vs. alpha(2)- isoforms have different external K(+) sensitivity and whether PLM and PKA activation affects the NKA affinity for K(+) in mouse cardiac myocytes. We measured the external [K(+)] dependence of the pump current generated by the ouabain-resistant NKA isoform in myocytes from wild-type (WT) mice (i.e., current due to NKA-alpha(1)) and mice in which the NKA isoforms have swapped ouabain affinities (alpha(1) is ouabain sensitive and alpha(2) is ouabain resistant) to assess current due to NKA-alpha(2). We found that NKA-alpha(1) has a higher affinity for external K(+) than NKA-alpha(2) [half-maximal pump activation (K(0.5)) = 1.5 +/- 0.1 vs. 2.9 +/- 0.3 mM]. The apparent external K(+) affinity of NKA was significantly lower in myocytes from WT vs. PLM-knockout mice (K(0.5) = 2.0 +/- 0.2 vs. 1.05 +/- 0.08 mM). However, PKA activation by isoproterenol (1 microM) did not alter the K(0.5) of NKA for external K(+) in WT myocytes. We conclude that 1) NKA-alpha(1) has higher affinity for K(+) than NKA-alpha(2) in cardiac myocytes, 2) PLM decreases the apparent external K(+) affinity of NKA, and 3) phosphorylation of PLM at the cytosolic domain does not alter apparent extracellular K(+) affinity of NKA.

Marinobufagenin enhances cardiac contractility in mice with ouabain-sensitive alpha1 Na+-K+-ATPase

Endogenous Na(+) pump inhibitors are thought to play important (patho)physiological roles and occur in two different chemical forms in the mammalian circulation: cardenolides, such as ouabain, and bufadienolides, such as marinobufagenin (MBG). Although all alpha Na(+)-K(+)-ATPase isoforms (alpha(1-4)) are sensitive to ouabain in most species, in rats and mice the ubiquitously expressed alpha(1) Na(+)-K(+)-ATPase is resistant to ouabain. We have previously shown that selective modification of the putative ouabain binding site of either the alpha(1) or alpha(2) Na(+)-K(+)-ATPase subunit in mice substantially alters the cardiotonic influence of exogenously applied cardenolides. To determine whether the ouabain binding site also interacts with MBG and if this interaction plays a functional role, we evaluated cardiovascular function in alpha(1)-resistant/alpha(2)-resistant (alpha(1)(R/R)alpha(2)(R/R)), alpha(1)-sensitive/alpha(2)-resistant (alpha(1)(S/S)alpha(2)(R/R)), and alpha(1)-resistant/alpha(2)-sensitive mice (alpha(1)(R/R)alpha(2)(S/S), wild type). Cardiovascular indexes were evaluated in vivo by cardiac catheterization at baseline and during graded infusions of MBG. There were no differences in baseline measurements of targeted mice, indicating normal hemodynamics and cardiac function. MBG at 0.025, 0.05, and 0.1 nmol*min(-1)*g body wt(-1) significantly increased cardiac performance to a greater extent in alpha(1)(S/S)alpha(2)(R/R) compared with alpha(1)(R/R)alpha(2)(R/R) and wild-type mice. The increase in LVdP/dt(max) in alpha(1)(S/S)alpha(2)(R/R) mice was greater at higher concentrations of MBG compared with both alpha(1)(R/R)alpha(2)(R/R) and alpha(1)(R/R)alpha(2)(S/S) mice (P < 0.05). These results suggest that MBG interacts with the ouabain binding site of the alpha(1) Na(+)-K(+)-ATPase subunit and can thereby influence cardiac inotropy.

Regulation of intracellular cholesterol distribution by Na/K-ATPase

Recent studies have ascribed many non-pumping functions to the Na/K-ATPase. We show here that graded knockdown of cellular Na/K-ATPase alpha1 subunit produces a parallel decrease in both caveolin-1 and cholesterol in light fractions of LLC-PK1 cell lysates. This observation is further substantiated by imaging analyses, showing redistribution of cholesterol from the plasma membrane to intracellular compartments in the knockdown cells. Moreover, this regulation is confirmed in alpha1(+/-) mouse liver. Functionally, the knockdown-induced redistribution appears to affect the cholesterol sensing in the endoplasmic reticulum, because it activates the sterol regulatory element-binding protein pathway and increases expression of hydroxymethylglutaryl-CoA reductase and low density lipoprotein receptor in the liver. Consistently, we detect a modest increase in hepatic cholesterol as well as a reduction in the plasma cholesterol. Mechanistically, alpha1(+/-) livers show increases in cellular Src and ERK activity and redistribution of caveolin-1. Although activation of Src is not required in Na/K-ATPase-mediated regulation of cholesterol distribution, the interaction between the Na/K-ATPase and caveolin-1 is important for this regulation. Taken together, our new findings demonstrate a novel function of the Na/K-ATPase in control of the plasma membrane cholesterol distribution. Moreover, the data also suggest that the plasma membrane Na/K-ATPase-caveolin-1 interaction may represent an important sensing mechanism by which the cells regulate the sterol regulatory element-binding protein pathway.

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.

Ouabain-Sensitive alpha1 Na,K-ATPase enhances natriuretic response to saline load

Na,K-ATPase is ubiquitously expressed and is essential for maintaining electrochemical and osmotic gradients. The alpha subunit of Na,K-ATPase is the receptor for cardiotonic steroids, which act through the ouabain-binding site and are important in cardiovascular regulation. Interestingly, the presence of endogenous Na,K-ATPase ligands has been implicated in the natriuretic response to perturbations such as hypertension and salt loading; therefore, it is important to characterize the role of the ouabain-binding sites in this context. Because the alpha1 isoform of mice and rats is relatively ouabain resistant, gene-targeting strategies were used to produce mice with reversed responses of the alpha1 and/or alpha2 isoforms to ouabain to assess for altered natriuretic responses to acute salt loading. Regardless of the sensitivity of the alpha2 isoform to ouabain, conferring ouabain sensitivity to alpha1 augmented the natriuretic response to an acute salt load. In addition, when endogenous Na,K-ATPase inhibitors were sequestered with an anti-digoxin antibody fragment, the sodium excretion rates in the ouabain-sensitive alpha1 isoform mice were equivalent to the ouabain-resistant alpha1 isoform mice. These data suggest that the ouabain-binding site of the alpha1 Na,K-ATPase can participate in the natriuretic response to a salt load by responding to endogenous Na,K-ATPase ligands.

ACTH-induced hypertension is dependent on the ouabain-binding site of the alpha2-Na+-K+-ATPase subunit

ACTH-induced-hypertension is commonly employed as a model of stress-related hypertension, and despite extensive investigation, the mechanisms underlying elevated blood pressure (BP) are not well understood. We have reported that ACTH treatment increases tail-cuff systolic pressure in wild-type mice but not in mutant mice expressing ouabain-resistant alpha(2)-Na(+)-K(+)-ATPase subunits (alpha2(R/R) mice). Since tail-cuff measurements involve restraint stress, the present study used telemetry to distinguish between an effect of ACTH on resting BP vs. an ACTH-enhanced stress response. We also sought to explore the mechanisms underlying ACTH-induced BP changes in mutant alpha2(R/R) mice vs. wild-type mice (ouabain-sensitive alpha(2)-Na(+)-K(+)-ATPase, alpha2(S/S) mice). Baseline BP was not different between the two genotypes, but after 5 days of ACTH treatment, BP increased in alpha2(S/S) (104.0 +/- 2.6 to 117.7 +/- 3.0 mmHg) but not in alpha2(R/R) mice (108.2 +/- 3.2 to 111.5 +/- 4.0 mmHg). To test the hypothesis that ACTH hypertension is related to inhibition of alpha(2)-Na(+)-K(+)-ATPase on vascular smooth muscle by endogenous cardiotonic steroids, we measured BP and regional blood flow. Results suggest a differential sensitivity of renal, mesenteric, and cerebral circulations to ACTH and that the response depends on the ouabain sensitivity of the alpha(2)-Na(+)-K(+)-ATPase. Baseline cardiac performance was elevated in alpha2(S/S) but not alpha2(R/R) mice. Overall, the data establish that the alpha(2)-Na(+)-K(+)-ATPase ouabain-binding site is of central importance in the development of ACTH-induced hypertension. The mechanism appears to be related to alterations in cardiac performance, and perhaps vascular tone in specific circulations, presumably caused by elevated levels of circulating cardiotonic steroids.

Na,K-ATPase and the role of alpha isoforms in behavior

The Na,K-ATPase is composed of multiple isoforms and the isoform distribution varies with the tissue and during development. The alpha1 isoform for example, is the major isoform in the kidney and many other tissues, while the alpha2 isoform is the predominate one in skeletal muscle. All three isoforms are found in the brain although in adult rodent brain, the alpha 3 isoform is located essentially in neurons while the alpha2 isoform is found in astrocytes and some limited neuronal populations. Interestingly the alpha 4 isoform is found exclusively in the mid region of the sperm tail. The distribution of the isoforms of the Na,K-ATPase has been extensively studied in many tissues and during development. The examples cited above provide some indication to the diversity of Na,K-ATPase isoform expression. In order to understand the significance of this distribution, we have developed animals which lack the alpha1, alpha2, and alpha 3 isoforms. It is anticipated that these studies will provide insight into the role that these isoforms play in driving various biological processes in specific tissues. Here we describe some of our studies which deal with the behavioral aspects of the alpha1, alpha2, and alpha 3 deficient mice, particularly those that are haploinsufficient in one isoform i.e. lacking one functional gene for the alpha1, alpha2, or alpha 3 isoforms. Such studies are important as two human diseases are associated with deficiency in the alpha2 and alpha 3 isoforms. These are Familial Hemiplegic Migraine type 2 and Rapid-Onset Dystonia Parkinsonism, these diseases result from alpha2 and alpha 3 isoform haploinsufficiency, respectively. We find that the haploinsufficiency of both alpha2 and alpha 3 isoforms result in behavioral defects.

KLF2 transcription factor modulates blood vessel maturation through smooth muscle cell migration

Vasculogenesis, angiogenesis, and maturation are three major phases of the development of blood vessels. Although many receptors required for blood vessel formation have been defined, the intracellular signal transduction pathways involved in vascular maturation remain unclear. KLF2(-/-) embryos fail to develop beyond 13.5 days because of a lack of blood vessel stabilization. The molecular mechanism of KLF2 function in embryonic vascular vessels is still largely unknown. Here we show a normal development pattern of endothelial cells in KLF2(-/-) embryos but a defect of smooth muscle cells at the dorsal side of the aorta. This phenotype results from arrested vascular maturation characterized by the failure of mural cells to migrate around endothelial cells. This migration defect is also observed when platelet-derived growth factor-B (PDGF) controlled migration is studied in murine embryonic fibroblast (MEF) cells from KLF2(-/-) animals. In addition, KLF2(-/-) MEFs exhibit a significant growth defect, indicating that KLF2 is required to maintain the viability of MEF cells. The PDGF signal is mediated through the Src signaling pathway, and a downstream target of KLF2 is sphingosine 1-phosphate receptor 1. These studies demonstrate that KLF2 is required for smooth muscle cell migration and elucidate a novel mechanism involving communication between PDGF and KLF2 in vascular maturation.

EKLF and KLF2 have compensatory roles in embryonic beta-globin gene expression and primitive erythropoiesis

The Krüppel-like C2/H2 zinc finger transcription factors (KLFs) control development and differentiation. Erythroid Krüppel-like factor (EKLF or KLF1) regulates adult beta-globin gene expression and is necessary for normal definitive erythropoiesis. KLF2 is required for normal embryonic Ey- and betah1-, but not adult betaglobin, gene expression in mice. Both EKLF and KLF2 play roles in primitive erythroid cell development. To investigate potential interactions between these genes, EKLF/KLF2 double-mutant embryos were analyzed. EKLF(-/-)KLF2(-/-) mice appear anemic at embryonic day 10.5 (E10.5) and die before E11.5, whereas single-knockout EKLF(-/-) or KLF2(-/-) embryos are grossly normal at E10.5 and die later than EKLF(-/-)KLF2(-/-) embryos. At E10.5, Ey- and betah1-globin mRNA is greatly reduced in EKLF(-/-)KLF2(-/-), compared with EKLF(-/-) or KLF2(-/-) embryos, consistent with the observed anemia. Light and electron microscopic analyses of E9.5 EKLF(-/-)KLF2(-/-) yolk sacs, and cytospins, indicate that erythroid and endothelial cells are morphologically more abnormal than in either single knockout. EKLF(-/-)KLF2(-/-) erythroid cells are markedly irregularly shaped, suggesting membrane abnormalities. EKLF and KLF2 may have coordinate roles in a common progenitor to erythroid and endothelial cells. The data indicate that EKLF and KLF2 have redundant functions in embryonic beta-like globin gene expression, primitive erythropoiesis, and endothelial development.

Deficiency in Na,K-ATPase alpha isoform genes alters spatial learning, motor activity, and anxiety in mice

Several disorders have been associated with mutations in Na,K-ATPase alpha isoforms (rapid-onset dystonia parkinsonism, familial hemiplegic migraine type-2), as well as reduction in Na,K-ATPase content (depression and Alzheimer's disease), thereby raising the issue of whether haploinsufficiency or altered enzymatic function contribute to disease etiology. Three isoforms are expressed in the brain: the alpha1 isoform is found in many cell types, the alpha2 isoform is predominantly expressed in astrocytes, and the alpha3 isoform is exclusively expressed in neurons. Here we show that mice heterozygous for the alpha2 isoform display increased anxiety-related behavior, reduced locomotor activity, and impaired spatial learning in the Morris water maze. Mice heterozygous for the alpha3 isoform displayed spatial learning and memory deficits unrelated to differences in cued learning in the Morris maze, increased locomotor activity, an increased locomotor response to methamphetamine, and a 40% reduction in hippocampal NMDA receptor expression. In contrast, heterozygous alpha1 isoform mice showed increased locomotor response to methamphetamine and increased basal and stimulated corticosterone in plasma. The learning and memory deficits observed in the alpha2 and alpha3 heterozygous mice reveal the Na,K-ATPase to be an important factor in the functioning of pathways associated with spatial learning. The neurobehavioral changes seen in heterozygous mice suggest that these mouse models may be useful in future investigations of the associated human CNS disorders.

Kruppel-like factor 2 regulates thymocyte and T-cell migration

Mammalian Kruppel-like transcription factors are implicated in regulating terminal differentiation of several tissue types. Deficiency in Kruppel-like factor (KLF) 2 (also known as LKLF) leads to a massive loss of the peripheral T-cell pool, suggesting KLF2 regulates T-cell quiescence and survival. Here we show, however, that KLF2 is essential for T-cell trafficking. KLF2-deficient (Klf2-/-) thymocytes show impaired expression of several receptors required for thymocyte emigration and peripheral trafficking, including the sphingosine-1-phosphate (S1P) receptor S1P1, CD62L and beta7 integrin. Furthermore, KLF2 both binds and transactivates the promoter for S1P1--a receptor that is critical for thymocyte egress and recirculation through peripheral lymphoid organs. Our findings suggest that KLF2 serves to license mature T cells for trafficking from the thymus and recirculation through secondary lymphoid tissues.

Ouabain inhibits tubuloglomerular feedback in mutant mice with ouabain-sensitive alpha1 Na,K-ATPase

Initiation of tubuloglomerular feedback (TGF) depends on Na-K-2Cl co-transport in the macula densa (MD), but it is less clear whether Na,K-ATPase is responsible for establishing the inward Na+ gradient. It has been proposed that apical colonic H,K-ATPase, perhaps in concert with the Na/H exchanger (NHE2), may account for MD Na+ exit in these cells. This study evaluated TGF responses by micropuncture in mutant mice with altered ouabain sensitivity of the alpha1 and alpha2 Na,K-ATPase isoforms. TGF responses in alpha1-sensitive/alpha2-resistant mice were inhibited by intravenous ouabain (control stop-flow pressure = 9.7 +/- 0.9 versus 1.6 +/- 0.5 mmHg with intravenous ouabain). Subsequent inclusion of cyclohexyladenosine (10 microM) in the tubule perfusate confirmed the ability of the afferent arteriole to contract in the presence of ouabain. In alpha1-resistant/alpha2-resistant mice, ouabain infusion had no effect on TGF responses. In separate experiments, loop of Henle perfusion with 50 microM ouabain decreased TGF responses (control stop-flow pressure) from 10.5 +/- 1.1 to 3.9 +/- 1.0 mmHg in alpha1-sensitive/alpha2-resistant mice but had no effect in alpha1-resistant/alpha2-resistant mice, and afferent arteriole responsiveness again was confirmed by cyclohexyladenosine. TGF responses in NHE2 and colonic H,K-ATPase knockout mice were not different from those of wild-type mice. These data indicate that TGF requires activity of the alpha1 Na,K-ATPase, presumably in the MD. Furthermore, the data show that neither NHE2 nor colonic H,K-ATPase is essential for initiation of TGF responses.

Up-regulation of the KLF2 transcription factor by fluid shear stress requires nucleolin

We have previously characterized the regulation of the KLF2 transcription factor gene by describing an induction complex that binds to and regulates its promoter. In the present study, by using DNA affinity chromatography and mass spectrometry, we have identified nucleolin as an additional protein that binds to a palindromic response region in the KLF2 promoter. The presence of nucleolin on the KLF2 promoter in macrophages was verified by electrophoretic mobility shift assays. Interestingly, in mouse and human endothelial cell lines, electrophoretic mobility shift assays and chromatin immunoprecipitation analyses indicated that nucleolin binds the KLF2 promoter only upon application of fluid shear stress. Pretreatment of the endothelial cells with LY294002, a specific inhibitor of phosphatidylinositol 3-kinase (PI3K), blocked the shear stress-induced binding of nucleolin to the promoter, demonstrating its PI3K-dependent regulation. Additionally, nucleolin exhibited dynamic flow-specific, PI3K-dependent alterations in size. Anti-nucleolin antibodies interacted with a 110-kDa form in static endothelial cells and with several catalytic forms that changed in abundance after the application of shear stress. Immunoprecipitation experiments demonstrated that fluid flow induced the interaction of nucleolin with the p85 regulatory subunit of PI3K. Finally, introduction of small interfering RNAs targeting the nucleolin genetic sequence selectively reduced nucleolin expression and was sufficient to block the induction of KLF2 by shear stress. These data support a general role for nucleolin in gene regulation and identify it as a novel factor involved in regulation of KLF2 expression.