The influence of Lyn kinase on Na,K-ATPase in porcine lens epithelium

Ion Transport Regulation by TRPV4 and TRPV1 in Lens and Ciliary Epithelium

Aside from a monolayer of epithelium at the anterior surface, the lens is formed by tightly compressed multilayers of fiber cells, most of which are highly differentiated and have a limited capacity for ion transport. Only the anterior monolayer of epithelial cells has high Na, K-ATPase activity. Because the cells are extensively coupled, the lens resembles a syncytium and sodium-potassium homeostasis of the entire structure is largely dependent on ion transport by the epithelium. Here we describe recent studies that suggest TRPV4 and TRPV1 ion channels activate signaling pathways that play an important role in matching epithelial ion transport activity with needs of the lens cell mass. A TRPV4 feedback loop senses swelling in the fiber mass and increases Na, K-ATPase activity to compensate. TRPV4 channel activation in the epithelium triggers opening of connexin hemichannels, allowing the release of ATP that stimulates purinergic receptors in the epithelium and results in the activation of Src family tyrosine kinases (SFKs) and SFK-dependent increase of Na, K-ATPase activity. A separate TRPV1 feedback loop senses shrinkage in the fiber mass and increases NKCC1 activity to compensate. TRPV1 activation causes calcium-dependent activation of a signaling cascade in the lens epithelium that involves PI3 kinase, ERK, Akt and WNK. TRPV4 and TRPV1 channels are also evident in the ciliary body where Na, K-ATPase is localized on one side of a bilayer in which two different cell types, non-pigmented and pigmented ciliary epithelium, function in a coordinated manner to secrete aqueous humor. TRPV4 and TRPV1 may have a role in maintenance of cell volume homeostasis as ions and water move through the bilayer.

Phosphorylation of Na+,K+-ATPase at Tyr10 of the α1-Subunit is Suppressed by AMPK and Enhanced by Ouabain in Cultured Kidney Cells

Na⁺,K⁺-ATPase (NKA) is essential for maintenance of cellular and whole-body water and ion homeostasis. In the kidney, a major site of ion transport, NKA consumes ~ 50% of ATP, indicating a tight coordination of NKA and energy metabolism. AMP-activated protein kinase (AMPK), a cellular energy sensor, regulates NKA by modulating serine phosphorylation of the α1-subunit, but whether it modulates other important regulatory phosphosites, such as Tyr10, is unknown. Using human kidney (HK-2) cells, we determined that the phosphorylation of Tyr10 was stimulated by the epidermal growth factor (EGF), which was opposed by inhibitors of Src kinases (PP2), tyrosine kinases (genistein), and EGF receptor (EGFR, gefitinib). AMPK activators AICAR and A-769662 suppressed the EGF-stimulated phosphorylation of EGFR (Tyr1173) and NKAα1 at Tyr10. The phosphorylation of Src (Tyr416) was unaltered by AICAR and increased by A-769662. Conversely, ouabain (100 nM), a pharmacological NKA inhibitor and a putative adrenocortical hormone, enhanced the EGF-stimulated Tyr10 phosphorylation without altering the phosphorylation of EGFR (Tyr1173) or Src (Tyr416). Ouabain (100–1000 nM) increased the ADP:ATP ratio, while it suppressed the lactate production and the oxygen consumption rate in a dose-dependent manner. Treatment with ouabain or gene silencing of NKAα1 or NKAα3 subunit did not activate AMPK. In summary, AMPK activators and ouabain had antagonistic effects on the phosphorylation of NKAα1 at Tyr10 in cultured HK-2 cells, which implicates a role for Tyr10 in coordinated regulation of NKA-mediated ion transport and energy metabolism.

Disruption of the lens circulation causes calcium accumulation and precipitates in connexin mutant mice

The lens is an avascular organ whose function and survival depend on an internal circulation system. Cx46fs380 mice model a human autosomal dominant cataract caused by a mutant lens connexin. In these mice, fiber cell connexin levels and gap junction coupling are severely decreased. The present studies were conducted to examine components of the lens circulation system that might be altered and contribute to the pathogenesis of cataracts. Lenses from wild-type mice and Cx46fs380 heterozygotes and homozygotes were studied at 2 months of age. Cx46fs380-expressing lens fiber cells were depolarized. Cx46fs380 lenses had increased intracellular hydrostatic pressure and concentrations of Na+ and Ca2+. The activity of epithelial Na+-K+-ATPase was decreased in Cx46fs380 lenses. All of these changes were more severe in homozygous than in heterozygous Cx46fs380 lenses. Cx46fs380 cataracts were stained by Alizarin red, a dye used to detect insoluble Ca2+. These data suggest that the lens internal circulation was disrupted by expression of Cx46fs380, leading to several consequences including accumulation of Ca2+ to levels so high that precipitates formed. Similar Ca2+-containing precipitates may contribute to cataract formation due to other genetic or acquired etiologies.

Regulation of renal function and structure by the signaling Na/K-ATPase

The Na/K-ATPase as an essential ion pump was discovered more than 50 years ago (Skou (1989) Biochim. Biophys. Acta 1000, 439-446; Feraille and Doucet (2001) Physiol. Rev. 81, 345-418). The signaling function of Na/K-ATPase has been gradually appreciated over the last 20 years, first from the studies of regulatory effects of ouabain on cardiac cell growth. Several reviews on this topic have been written during the last few years (Schoner and Scheiner-Bobis (2007) Am. J. Physiol. Cell. Physiol. 293, C509-C536; Xie and Cai (2003) Mol. Interv. 3, 157 - 168; Bagrov et al. (2009) Pharmacol. Rev. 61, 9-38; Tian and Xie (2008) Physiology 23, 205-211; Fontana et al. (2013) FEBS J. 280, 5450-5455; Blanco and Wallace (2013) Am. J. Physiol. Renal Physiol. 305, F797-F812). This article will focus on the molecular mechanism of Na/K-ATPase-mediated signal transduction and its potential regulatory role in renal physiology and diseases.

Distinct roles for N-Cadherin linked c-Src and fyn kinases in lens development

BACKGROUND

Src family tyrosine kinases (SFKs) are often coincidently expressed but few studies have dissected their individual functions in the same cell during development. Using the classical embryonic lens as our model, we investigated SFK signaling in the regulation of both differentiation initiation and morphogenesis, and the distinct functions of c-Src and Fyn in these processes.

RESULTS

Blocking SFK activity with the highly specific inhibitor PP1 induced initiation of the lens differentiation program but blocked lens fiber cell elongation and organization into mini lens-like structures called lentoids. These dichotomous roles for SFK signaling were discovered to reflect distinct functions of c-Src and Fyn and their differentiation-state-specific recruitment to and action at N-cadherin junctions. c-Src was highly associated with the nascent N-cadherin junctions of undifferentiated lens epithelial cells. Its siRNA knockdown promoted N-cadherin junctional maturation, blocked proliferation, and induced lens cell differentiation. In contrast, Fyn was recruited to mature N-cadherin junctions of differentiating lens cells and siRNA knockdown suppressed differentiation-specific gene expression and blocked morphogenesis.

CONCLUSIONS

Through inhibition of N-cadherin junction maturation, c-Src promotes lens epithelial cell proliferation and the maintenance of the lens epithelial cell undifferentiated state, while Fyn, signaling downstream of mature N-cadherin junctions, promotes lens fiber cell morphogenesis.

Comparative properties of caveolar and noncaveolar preparations of kidney Na+/K+-ATPase

To evaluate previously proposed functions of renal caveolar Na⁺/K⁺-ATPase, we modified the standard procedures for the preparation of the purified membrane-bound kidney enzyme, separated the caveolar and noncaveolar pools, and compared their properties. While the subunits of Na⁺/K⁺-ATPase (α,β,γ) constituted most of the protein content of the noncaveolar pool, the caveolar pool also contained caveolins and major caveolar proteins annexin-2 tetramer and E-cadherin. Ouabain-sensitive Na⁺/K⁺-ATPase activities of the two pools had similar properties and equal molar activities, indicating that the caveolar enzyme retains its ion transport function and does not contain nonpumping enzyme. As minor constituents, both caveolar and noncaveolar pools also contained Src, EGFR, PI3K, and several other proteins known to be involved in stimulous-induced signaling by Na⁺/K⁺-ATPase, indicating that signaling function is not limited to the caveolar pool. Endogenous Src was active in both pools but was not further activated by ouabain, calling into question direct interaction of Src with native Na⁺/K⁺-ATPase. Chemical cross-linking, co-immunoprecipitation, and immunodetection studies showed that in the caveolar pool, caveolin-1 oligomers, annexin-2 tetramers, and oligomers of the α,β,γ-protomers of Na⁺/K⁺-ATPase form a large multiprotein complex. In conjunction with known roles of E-cadherin and the β-subunit of Na⁺/K⁺-ATPase in cell adhesion and noted intercellular β,β-contacts within the structure of Na⁺/K⁺-ATPase, our findings suggest that interacting caveolar Na⁺/K⁺-ATPases located at renal adherens junctions maintain contact of two adjacent cells, conduct essential ion pumping, and are capable of locus-specific signaling in junctional cells.

Purinergic agonists stimulate lens Na-K-ATPase-mediated transport via a Src tyrosine kinase-dependent pathway

The Na-K-ATPase is vital for maintenance of lens transparency. Past studies using intact lens suggested the involvement of tyrosine kinases in short-term regulation of Na-K-ATPase. Furthermore, in vitro phosphorylation of a lens epithelial membrane preparation by Src family kinases (SFKs), a family of nonreceptor tyrosine kinases, resulted in modification of Na-K-ATPase activity. Here, the effect of purinergic agonists, ATP and UTP, on Na-K-ATPase function and SFK activation was examined in the rabbit lens. Na-K-ATPase function was examined using two different approaches, measurement of ouabain-sensitive potassium ((86)Rb) uptake by the intact lens, and Na-K-ATPase activity in lens epithelial homogenates. ATP and UTP caused a significant increase in ouabain-sensitive potassium ((86)Rb) uptake. Na-K-ATPase activity was increased in the epithelium of lenses pretreated with ATP. Lenses treated with ATP or UTP displayed activation of SFKs as evidenced by increased Western blot band density of active SFK (phosphorylated at the active loop Y416) and decreased band density of inactive SFKs (phosphorylated at the COOH terminal). A single PY416-Src immunoreactive band at approximately 60 kDa was observed, suggesting not all Src family members are activated. Immunoprecipitation studies showed that band density of active Src, and to a lesser extent active Fyn, was significantly increased, while active Yes did not change. Preincubation of the lenses with SFK inhibitor PP2 abolished the ATP-induced increase in ouabain-sensitive potassium ((86)Rb) uptake. The results suggest selective activation of Src and/or Fyn is part of a signaling mechanism initiated by purinergic agonists that increases Na-K-ATPase-mediated transport in the organ-cultured lens.

Regulation of potassium transport in human lens epithelial cells

The major K influx pathways and their response to thiol modification by N-ethylmaleimide (NEM) and protein kinase and phosphatase inhibitors were characterized in human lens epithelial B3 (HLE-B3) cells with Rb as K congener. Ouabain (0.1 mM) and bumetanide (5 microM) discriminated between the Na/K pump ( approximately 35% of total Rb influx) and Na-K-2Cl cotransport (NKCC) ( approximately 50%). Cl-replacement with nitrate or sulfamate revealed <10% residual [ouabain+bumetanide]-insensitive K-Cl cotransport (KCC). At 0.3-0.5 mM, NEM stimulated the Na/K pump by 2-fold independent of external Na, KCC between 2 and 4-fold, and abolished approximately 90% of NKCC. Calyculin-A, a serine/threonine protein phosphatase-1 inhibitor, did not affect NKCC but inhibited KCC, whereas 10 microM staurosporine, a serine/threonine kinase inhibitor, abolished NKCC, and stimulated KCC only when followed by NEM treatment. The tyrosine-kinase inhibitor genistein, at concentrations >100 microM, activated the Na/K pump and abolished NKCC but did not affect KCC. The data suggest at least partial inverse regulation of KCC and NKCC in HLE-B3 cells by signaling cascades involving serine, threonine and tyrosine phosphorylation/dephosphorylation equilibria.

Novel regulation of Na, K-ATPase by Src tyrosine kinases in cortical neurons

The Na+, K+-ATPase or Na+, K+-pump plays a critical role in ion homeostasis and many cellular events. The Na+, K+-pump activity is regulated by serine/threonine phosphorylation, the role of tyrosine kinases in the regulation, however, is obscure. We now present novel evidence showing that tyrosine phosphorylation activates the Na+, K+-pump in cortical neurons. The electrogenic activity of the Na+, K+-pump was measured using whole-cell voltage clamp. A tonic activity was revealed by an inward current induced by the specific inhibitor ouabain or strophanthidin; an outward current due to activation of the pump was triggered by raising extracellular K+. The inward and outward currents were attenuated by the tyrosine kinase inhibitor genistein, herbimycin A, or lavendustin A, while blocking tyrosine phosphatases increased the pump current. Down-regulation of the pump current was also seen with the Src inhibitor PP1 and intracellularly applied anti-Lyn or anti-Yes antibody. Consistently, intracellular application of Lyn kinase up-regulated the pump current. Immunoprecipitation and western blotting showed tyrosine phosphorylation and a direct interaction between Lyn and the alpha3 subunit of the Na+, K+-pump. The tyrosine phosphorylation of the alpha3 subunit was reduced by serum deprivation. These data suggest that the Na+, K+-ATPase activity in central neurons is regulated by specific Src tyrosine kinases via a protein-protein mechanism and may play a role in apoptosis.

Dysregulation of Na+/K+ ATPase by amyloid in APP+PS1 transgenic mice

Background

The pathology of Alzheimer's disease (AD) is comprised of extracellular amyloid plaques, intracellular tau tangles, dystrophic neurites and neurodegeneration. The mechanisms by which these various pathological features arise are under intense investigation. Here, expanding upon pilot gene expression studies, we have further analyzed the relationship between Na+/K+ ATPase and amyloid using APP+PS1 transgenic mice, a model that develops amyloid plaques and memory deficits in the absence of tangle formation and neuronal or synaptic loss.

Results

We report that in addition to decreased mRNA expression, there was decreased overall Na+/K+ ATPase enzyme activity in the amyloid-containing hippocampi of the APP+PS1 mice (although not in the amyloid-free cerebellum). In addition, dual immunolabeling revealed an absence of Na+/K+ ATPase staining in a zone surrounding congophilic plaques that was occupied by dystrophic neurites. We also demonstrate that cerebral Na+/K+ ATPase activity can be directly inhibited by high concentrations of soluble Aβ.

Conclusions

The data suggest that the reductions in Na+/K+ ATPase activity in Alzheimer tissue may not be purely secondary to neuronal loss, but may results from direct effects of amyloid on this enzyme. This disruption of ion homeostasis and osmotic balance may interfere with normal electrotonic properties of dendrites, blocking intraneuronal signal processing, and contribute to neuritic dystrophia. These results suggest that therapies aimed at enhancing Na+/K+ ATPase activity in AD may improve symptoms and/or delay disease progression.

Expression, regulation and function of Na,K-ATPase in the lens

Na,K-ATPase is responsible for maintaining the correct concentrations of sodium and potassium in lens cells. Na,K-ATPase activity is different in the two cell types that make up the lens, epithelial cells and fibers; specific activity in the epithelium is higher than in fibers. In some parts of the fiber mass Na,K-ATPase activity is barely detectable. There is a large body of evidence that suggests Na,K-ATPase-mediated ion transport by the epithelium contributes significantly to the regulation of ionic composition in the entire lens. In some species different Na,K-ATPase isoforms are present in epithelium and fibers but in general, fibers and epithelium express a similar amount of Na,K-ATPase protein. Turnover of Na,K-ATPase by protein synthesis may contribute to preservation of high Na,K-ATPase activity in the epithelium. In ageing lens fibers, oxidation, and glycation may decrease Na,K-ATPase activity. Na,K-ATPase activity in lens fibers and epithelium also may be subject to regulation as the result of protein tyrosine phosphorylation. Moreover, activation of G protein-coupled receptors by agonists such as endothelin-1 elicits changes of Na,K-ATPase activity. The asymmetrical distribution of Na,K-ATPase activity in the epithelium and fibers may contribute to ionic currents that flow in and around the lens. Studies on human cataract and experimental cataract in animals reveal changes of Na,K-ATPase activity but no clear pattern is evident. However, there is a convincing link between abnormal elevation of lens sodium and the opacification of the lens cortex that occurs in age-related human cataract.

The influence of protein tyrosine phosphatase-1B on Na,K-ATPase activity in lens

The abnormal sodium content of many cataracts suggests Na,K-ATPase is vital for maintenance of eye lens transparency. Since tyrosine phosphorylation is considered a possible regulatory mechanism for Na,K-ATPase, experiments were conducted to test the influence of protein tyrosine phosphatase-1B (PTP-1B) on Na,K-ATPase activity. Membrane material was isolated separately from porcine lens epithelium and fiber cells. Tyrosine phosphoproteins, Na,K-ATPase alpha1 polypeptide and PTP-1B were examined by Western blot. Na,K-ATPase activity was determined by measuring ATP hydrolysis in the presence or absence of ouabain. Western blot analysis revealed tyrosine phosphorylation of multiple membrane proteins in both lens cell types, the differentiated fiber cells and non-differentiated epithelium. When membrane material was subjected to immunoprecipitation using an antibody directed against Na,K-ATPase alpha1, a colocalized phosphotyrosine band was detected in lens fibers but not epithelium. Incubation with PTP-1B caused a approximately 50% increase of Na,K-ATPase activity in fiber membrane material. Na,K-ATPase activity in lens epithelium membrane material was not significantly altered by PTP-1B treatment even though PTP-1B was demonstrated to cause dephosphorylation of multiple membrane proteins in the epithelium as well as fibers. While endogenous PTP-1B was detected in both cell types, endogenous tyrosine phosphatase activity was low in both epithelium and fiber membrane material. The results illustrate endogenous tyrosine phosphorylation of Na,K-ATPase alpha1 polypeptide in fibers. Na,K-ATPase alpha1 in lens fibers may be a potential target for PTP-1B.

Organ hypertrophic signaling within caveolae membrane subdomains triggered by ouabain and antagonized by PST 2238

In addition to inhibition of the Na-K ATPase, ouabain activates a signal transduction function, triggering growth and proliferation of cultured cells even at nanomolar concentrations. An isomer of ouabain (EO) circulates in mammalians at subnanomolar concentrations, and increased levels are associated with cardiac hypertrophy and hypertension. We present here a study of cardiac and renal hypertrophy induced by ouabain infused into rats for prolonged periods and relate this effect to the recently described ouabain-induced activation of the Src-EGFr-ERK signaling pathway. Ouabain infusion into rats (15 microg/kg/day for 18 weeks) doubled plasma ouabain levels from 0.3 to 0.7 nm and increased blood pressure by 20 mm Hg (p < 0.001), cardiac left ventricle (+11%, p < 0.05), and kidney weight (+9%, p < 0.01). These effects in vivo are associated with a significant enrichment of alpha1, beta1, gammaa Na-K ATPase subunits together with Src and EGFr in isolated renal caveolae membranes and activation of ERK1/2. In caveolae, direct Na-K ATPase/Src interactions can be demonstrated by co-immunoprecipitation. The interaction is amplified by ouabain, at a high affinity binding site, detectable in caveolae but not in total rat renal membranes. The high affinity site for ouabain is associated with Src-dependent tyrosine phosphorylation of rat alpha1 Na-K ATPase. The antihypertensive compound, PST 2238, antagonized all ouabain-induced effects at 10 microg/kg/day in vivo or 10(-10)-10(-8) m in vitro. These findings provide a molecular mechanism for the in vivo pro-hypertrophic and hypertensinogenic activity of ouabain, or by analogy those of EO in humans. They also explain the pharmacological basis for PST 2238 treatment.