A Cataract-Causing Mutation in the TRPM3 Cation Channel Disrupts Calcium Dynamics in the Lens

TRPM3 belongs to the melastatin sub-family of transient receptor potential (TRPM) cation channels and has been shown to function as a steroid-activated, heat-sensitive calcium ion (Ca2+) channel. A missense substitution (p.I65M) in the TRPM3 gene of humans (TRPM3) and mice (Trpm3) has been shown to underlie an inherited form of early-onset, progressive cataract. Here, we model the pathogenetic effects of this cataract-causing mutation using 'knock-in' mutant mice and human cell lines. Trpm3 and its intron-hosted micro-RNA gene (Mir204) were strongly co-expressed in the lens epithelium and other non-pigmented and pigmented ocular epithelia. Homozygous Trpm3-mutant lenses displayed elevated cytosolic Ca2+ levels and an imbalance of sodium (Na+) and potassium (K+) ions coupled with increased water content. Homozygous TRPM3-mutant human lens epithelial (HLE-B3) cell lines and Trpm3-mutant lenses exhibited increased levels of phosphorylated mitogen-activated protein kinase 1/extracellular signal-regulated kinase 2 (MAPK1/ERK2/p42) and MAPK3/ERK1/p44. Mutant TRPM3-M65 channels displayed an increased sensitivity to external Ca2+ concentration and an altered dose response to pregnenolone sulfate (PS) activation. Trpm3-mutant lenses shared the downregulation of genes involved in insulin/peptide secretion and the upregulation of genes involved in Ca2+ dynamics. By contrast, Trpm3-deficient lenses did not replicate the pathophysiological changes observed in Trpm3-mutant lenses. Collectively, our data suggest that a cataract-causing substitution in the TRPM3 cation channel elicits a deleterious gain-of-function rather than a loss-of-function mechanism in the lens.

Patch clamp studies on TRPV4-dependent hemichannel activation in lens epithelium

ATP release from the lens via hemichannels has been explained as a response to TRPV4 activation when the lens is subjected to osmotic swelling. To explore the apparent linkage between TRPV4 activation and connexin hemichannel opening we performed patch-clamp recordings on cultured mouse lens epithelial cells exposed to the TRPV4 agonist GSK1016790A (GSK) in the presence or absence of the TRPV4 antagonist HC067047 (HC). GSK was found to cause a fast, variable and generally large non-selective increase of whole cell membrane conductance evident as a larger membrane current (Im) over a wide voltage range. The response was prevented by HC. The GSK-induced Im increase was proportionally larger at negative voltages and coincided with fast depolarization and the simultaneous disappearance of an outward current, likely a K⁺ current. The presence of this outward current in control conditions appeared to be a reliable predictor of a cell's response to GSK treatment. In some studies, recordings were obtained from single cells by combining cell-attached and whole-cell patch clamp configurations. This approach revealed events with a channel conductance 180-270 pS following GSK application through the patch pipette on the cell-attached side. The findings are consistent with TRPV4-dependent opening of Cx43 hemichannels.

NMDA receptor inhibition prevents intracellular sodium elevations in human olfactory neuroepithelial precursors derived from bipolar patients

Dysregulation of ion flux across membranes and glutamate-induced excitotoxicity appear to be important pathophysiologic abnormalities in bipolar illness. Understanding ion control and responses to ionic stress is important to decipher the pathogenesis of this disorder. Monensin alone significantly increased [Na]_i in ONPs from bipolar individuals (5.08 ± 0.71 vs baseline 3.13 ± 0.93, P = 0.03) and AP5 had no effect (2.0 ± 1.2 vs baseline 3.13 ± 0.93, P = 0.27). However, the combination of AP5 and monensin resulted in normalization of [Na]_i (3.25 ± 1.28 vs baseline 3.13 ± 0.93, P = 0.89). This effect was not observed in cells from non-bipolar individuals (monensin alone, 1.72 ± 1.10 vs baseline 2.42 ± 1.80, P = 0.25; AP5 alone, 1.37 ± 0.74 vs baseline 2.42 ± 1.80; AP5 combined with monensin, 1.53 ± 0.98 vs baseline 2.42 ± 1.80, P = 0.31). Sodium regulation is central to neuronal function and may be disturbed in patients with bipolar disorder. Monensin is an ionophore, meaning that it incorporates itself into the membrane and allows sodium to enter independent of cellular membrane proteins. While the mechanism remains obscure, the observation that the NMDA receptor antagonist, AP5, normalizes [Na]_i only in olfactory neuroepithelial precursors obtained from bipolar illness may provide novel insights into ion regulation in tissues from subjects with bipolar illness.

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.

Signaling Between TRPV1/TRPV4 and Intracellular Hydrostatic Pressure in the Mouse Lens

Purpose

The lens uses feedback to maintain zero pressure in its surface cells. Positive pressures are detected by transient receptor potential vanilloid (TRPV4), which initiates a cascade that reduces surface cell osmolarity. The first step is opening of gap junction hemichannels. One purpose of the current study was to identify the connexin(s) in the hemichannels. Negative pressures are detected by TRPV1, which initiates a cascade that increases surface osmolarity. The increase in osmolarity was initially reported to be through inhibition of Na/K ATPase activity, but a recent study reported it was through stimulation of Na/K/2Cl (NKCC) cotransport. A second purpose of this study was to reconcile these two reports.

Methods

Intracellular hydrostatic pressures were measured using a microelectrode/manometer system. Lenses from TRPV1 or Cx50 null mice were studied. Specific inhibitors of Cx50 gap junction channels, NKCC, and Akt were used.

Results

Either knockout of Cx50 or blockade of Cx50 channels completely eliminated the response to positive surface pressures. Knockout of Cx50 also caused a positive drift in surface pressure. The short-term (∼20-minute) response to negative surface pressures was eliminated by blockade of NKCC, but a long-term (∼4-hour) response restored pressure to zero. Both short- and long-term responses were eliminated by knockout of TRPV1 or inhibition of Akt.

Conclusions

Hemichannels made from Cx50 are required for the response to positive surface pressures. Negative surface pressures first activate NKCC, but a backup system is inhibition of Na/K ATPase activity. Both responses are initiated by TRPV1 and go through PI3K/Akt before branching.

TRPV1 activation stimulates NKCC1 and increases hydrostatic pressure in the mouse lens

The porcine lens response to a hyperosmotic stimulus involves an increase in the activity of an ion cotransporter sodium-potassium/two-chloride cotransporter 1 (NKCC1). Recent studies with agonists and antagonists pointed to a mechanism that appears to depend on activation of transient receptor potential vanilloid 1 (TRPV1) ion channels. Here, we compare responses in lenses and cultured lens epithelium obtained from TRPV1^-/- and wild type (WT) mice. Hydrostatic pressure (HP) in lens surface cells was determined using a manometer-coupled microelectrode approach. The TRPV1 agonist capsaicin (100 nM) caused a transient HP increase in WT lenses that peaked after ∼30 min and then returned toward baseline. Capsaicin did not cause a detectable change of HP in TRPV1^-/- lenses. The NKCC inhibitor bumetanide prevented the HP response to capsaicin in WT lenses. Potassium transport was examined by measuring Rb⁺ uptake. Capsaicin increased Rb⁺ uptake in cultured WT lens epithelial cells but not in TRPV1^-/- cells. Bumetanide, A889425, and the Akt inhibitor Akti prevented the Rb⁺ uptake response to capsaicin. The bumetanide-sensitive (NKCC-dependent) component of Rb⁺ uptake more than doubled in response to capsaicin. Capsaicin also elicited rapid (<2 min) NKCC1 phosphorylation in WT but not TRPV1^-/- cells. HP recovery was shown to be absent in TRPV1^-/- lenses exposed to hyperosmotic solution. Bumetanide and Akti prevented HP recovery in WT lenses exposed to hyperosmotic solution. Taken together, responses to capsaicin and hyperosmotic solution point to a functional role for TRPV1 channels in mouse lens. Lack of NKCC1 phosphorylation and Rb⁺ uptake responses in TRPV1^-/- mouse epithelium reinforces the notion that a hyperosmotic challenge causes TRPV1-dependent NKCC1 activation. The results are consistent with a role for the TRPV1-activated signaling pathway leading to NKCC1 stimulation in lens osmotic homeostasis.

Activation of TRPV1 channels leads to stimulation of NKCC1 cotransport in the lens

Lens ion homeostasis is crucial in maintaining water content and, in turn, refractive index and transparency of the multicellular syncytium-like structure. New information is emerging on the regulation of ion transport in the lens by mechanisms that rely on transient receptor potential vanilloid (TRPV) ion channels. We found recently that TRPV1 activation leads to Ca²⁺/PKC-dependent ERK1/2 signaling. Here, we show that the TRPV1 agonist capsaicin (100 nM) and hyperosmotic solution (350 vs. 300 mosM) each caused an increase of bumetanide-inhibitable Rb uptake by intact porcine lenses and Na-K-2Cl cotransporter 1 (NKCC1) phosphorylation in the lens epithelium. The TRPV1 antagonist A889425 (1 µM) abolished the increases of Rb uptake and NKCC1 phosphorylation in response to hyperosmotic solution. Exposing lenses to hyperosmotic solution in the presence of MEK/ERK inhibitor U0126 (10 µM) or the with-no-lysine kinase (WNK) inhibitor WNK463 (1 µM) also prevented NKCC1 phosphorylation and the Rb uptake responses to hyperosmotic solution. WNK463 did not prevent the increase in ERK1/2 phosphorylation that occurs in response to capsaicin or hyperosmotic solution, suggesting that ERK1/2 activation occurs before WNK activation in the sequence of signaling events. Taken together, the evidence indicates that activation of TRPV1 is a critical early step in a signaling mechanism that responds to a hyperosmotic stimulus, possibly lens shrinkage. By activating ERK1/2 and WNK, TRPV1 activation leads to NKCC1 phosphorylation and stimulation of NKCC1-mediated ion transport.

Correcting deregulated Fxyd1 expression rescues deficits in neuronal arborization and potassium homeostasis in MeCP2 deficient male mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the MECP2 gene. In the absence of MeCP2, expression of FXYD domain-containing transport regulator 1 (FXYD1) is deregulated in the frontal cortex (FC) of mice and humans. Because Fxyd1 is a membrane protein that controls cell excitability by modulating Na⁺, K⁺-ATPase activity (NKA), an excess of Fxyd1 may reduce NKA activity and contribute to the neuronal phenotype of Mecp2 deficient (KO) mice. To determine if Fxyd1 can rescue these RTT deficits, we studied the male progeny of Fxyd1 null males bred to heterozygous Mecp2 female mice. Maximal NKA enzymatic activity was not altered by the loss of MeCP2, but it increased in mice lacking one Fxyd1 allele, suggesting that NKA activity is under Fxyd1 inhibitory control. Deletion of one Fxyd1 allele also prevented the increased extracellular potassium (K⁺) accumulation observed in cerebro-cortical neurons from Mecp2 KO animals in response to the NKA inhibitor ouabain, and rescued the loss of dendritic arborization observed in FC neurons of Mecp2 KO mice. These effects were gene-dose dependent, because the absence of Fxyd1 failed to rescue the MeCP2-dependent deficits, and mimicked the effect of MeCP2 deficiency in wild-type animals. These results indicate that excess of Fxyd1 in the absence of MeCP2 results in deregulation of endogenous K⁺ conductances functionally associated with NKA and leads to stunted neuronal growth.

TRPV1-dependent ERK1/2 activation in porcine lens epithelium

Recently we determined that the Transient Receptor Potential Vanilloid 4 ion channel (TRPV4) has a crucial signaling role in a pathway that regulates various aspects of lens epithelium function. Here, we report on a different TRPV channel, TRPV1, in porcine lens. The presence of TRPV1 in the lens was evident from RT-PCR studies and Western blot analysis of MAPK signaling pathway activation caused by the TRPV1 agonist capsaicin. TRPV1 mRNA was detected in the epithelium of porcine as well as human lens. Transient ERK1/2 and p38 MAPK phosphorylation was detected within 1 min in the epithelium isolated from intact porcine lenses exposed to capsaicin (100 nM), a selective TRPV1 agonist, and the response was significantly inhibited by A889245 (1.0 μM), a TRPV1 antagonist. A similar ERK 1/2 and p38 response in the epithelium, also inhibitable by A889245, was evident in lenses treated with hyperosmotic solution (350 vs 300 mOsm). Lenses pre-treated with either the cytosolic Ca²⁺ chelator BAPTA-AM or the PKC inhibitor sotrastaurin (1.0 μM) had a diminished ERK1/2 activation response to capsaicin and hyperosmotic solution. Taken together the findings support the notion that TRPV1 functions as a plasma membrane ion channel that, when activated, permits the entry of extracellular calcium into the lens epithelium, leading to activation of PKC, ERK1/2 and p38 MAPK. It is significant that the findings confirm earlier proposals that hyperosmotic stress is linked to TRPV1 channel activation in the mouse lens. Further studies are ongoing to determine what functional changes are triggered by the TRPV1-linked signaling pathways and how they might relate to lens volume homeostasis.

A Role for Calcium-Activated Adenylate Cyclase and Protein Kinase A in the Lens Src Family Kinase and Na,K-ATPase Response to Hyposmotic Stress

Purpose

Na,K-ATPase activity in lens epithelium is subject to control by Src family tyrosine kinases (SFKs). Previously we showed hyposmotic solution causes an SFK-dependent increase in Na,K-ATPase activity in the epithelium. Here we explored the role of cAMP in the signaling mechanism responsible for the SFK and Na,K-ATPase response.

Methods

Intact porcine lenses were exposed to hyposmotic Krebs solution (200 mOsm) then the epithelium was assayed for cAMP, SFK phosphorylation (activation) or Na,K-ATPase activity.

Results

An increase of cAMP was observed in the epithelium of lenses exposed to hyposmotic solution. In lenses exposed to hyposmotic solution SFK phosphorylation in the epithelium approximately doubled as did Na,K-ATPase activity and both responses were prevented by H89, a protein kinase A inhibitor. The magnitude of the SFK response to hyposmotic solution was reduced by a TRPV4 antagonist HC067047 added to prevent TRPV4-mediated calcium entry, and by a cytoplasmic Ca2+ chelator BAPTA-AM. The Na,K-ATPase activity response in the epithelium of lenses exposed to hyposmotic solution was abolished by BAPTA-AM. As a direct test of cAMP-dependent SFK activation, intact lenses were exposed to 8-pCPT-cAMP, a cell-permeable cAMP analog. 8-pCPT-cAMP caused robust SFK activation. Using Western blot, two calcium-activated adenylyl cyclases, ADCY3 and ADCY8, were detected in lens epithelium.

Conclusions

Calcium-activated adenylyl cyclases are expressed in the lens epithelium and SFK activation is linked to a rise of cAMP that occurs upon hyposmotic challenge. The findings point to cAMP as a link between TRPV4 channel-mediated calcium entry, SFK activation, and a subsequent increase of Na,K-ATPase activity.

Src Family Kinase Links Insulin Signaling to Short Term Regulation of Na,K-ATPase in Nonpigmented Ciliary Epithelium

Insulin has been shown to elicit changes of Na,K-ATPase activity in various tissues. Na,K-ATPase in the nonpigmented ciliary epithelium (NPE) plays a role in aqueous humor secretion and changes of Na,K-ATPase activity impact the driving force. Because we detect a change of NPE Na,K-ATPase activity in response to insulin, studies were carried out to examine the response mechanism. Ouabain-sensitive rubidium (Rb) uptake by cultured NPE cells, measured as a functional index of Na,K-ATPase-mediated inward potassium transport, was found to increase in cells exposed for 5 min to insulin. The maximally effective concentration was 100 nM. An intrinsic increase of Na,K-ATPase activity evident as a >2-fold increase in the rate of ouabain-sensitive ATP hydrolysis in homogenates obtained from cells exposed to 100 nM insulin for 5 min was also observed. Insulin-treated cells exhibited Akt, Src family kinase (SFK), ERK1/2, and p38 activation, all of which were prevented by a pI3 kinase inhibitor LY294002. The Rb uptake and Na,K-ATPase activity response to insulin both were abolished by PP2, an SFK inhibitor which also prevented p38 and ERK1/2 but not Akt activation. The Akt inhibitor MK-2206 did not change the Na,K-ATPase response to insulin. The findings suggest insulin activates pI3K-dependent Akt and SFK signaling pathways that are separate. ERK1/2 and p38 activation is secondary to and dependent on SFK activation. The increase of Na,K-ATPase activity is dependent on activation of the SFK pathway. The findings are consistent with previous studies that indicate a link between Na,K-ATPase activity and SFK signaling. J. Cell. Physiol. 232: 1489-1500, 2017. © 2016 Wiley Periodicals, Inc.

Feedback Regulation of Intracellular Hydrostatic Pressure in Surface Cells of the Lens

In wild-type lenses from various species, an intracellular hydrostatic pressure gradient goes from ∼340 mmHg in central fiber cells to 0 mmHg in surface cells. This gradient drives a center-to-surface flow of intracellular fluid. In lenses in which gap-junction coupling is increased, the central pressure is lower, whereas if gap-junction coupling is reduced, the central pressure is higher but surface pressure is always zero. Recently, we found that surface cell pressure was elevated in PTEN null lenses. This suggested disruption of a feedback control system that normally maintained zero surface cell pressure. Our purpose in this study was to investigate and characterize this feedback control system. We measured intracellular hydrostatic pressures in mouse lenses using a microelectrode/manometer-based system. We found that all feedback went through transport by the Na/K ATPase, which adjusted surface cell osmolarity such that pressure was maintained at zero. We traced the regulation of Na/K ATPase activity back to either TRPV4, which sensed positive pressure and stimulated activity, or TRPV1, which sensed negative pressure and inhibited activity. The inhibitory effect of TRPV1 on Na/K pumps was shown to signal through activation of the PI3K/AKT axis. The stimulatory effect of TRPV4 was shown in previous studies to go through a different signal transduction path. Thus, there is a local two-legged feedback control system for pressure in lens surface cells. The surface pressure provides a pedestal on which the pressure gradient sits, so surface pressure determines the absolute value of pressure at each radial location. We speculate that the absolute value of intracellular pressure may set the radial gradient in the refractive index, which is essential for visual acuity.

The Significance of TRPV4 Channels and Hemichannels in the Lens and Ciliary Epithelium

To function normally, all cells must maintain ion homeostasis, establish a membrane potential, and regulate water content. These actions require active Na-K transport provided by Na,K-ATPase. The lens, however, is made up almost entirely of fiber cells that have little or no Na,K-ATPase activity. Lens ion and water homeostasis rely on Na,K-ATPase activity in a small number of cells at the periphery of epithelium monolayer. Therefore, the function of the epithelium must be integrated with the needs of the fiber mass. This suggests that a remote control mechanism may adjust Na,K-ATPase activity to match increases or decreases of ion leakage, which may occur a considerable distance away. Here, we review evidence that TRPV4 channels in the epithelium become activated when the lens is subjected to osmotic- or damage-induced swelling. This triggers a chain of events in the lens epithelium that opens connexin hemichannels, allowing ATP release that stimulates purinergic receptors, activates Src family tyrosine kinases, and increases Na,K-ATPase activity. Recent studies also revealed functional connexin hemichannels along with TRPV4 channels in nonpigmented ciliary epithelial (NPE) cells that secrete aqueous humor into the eye. Because TRPV4 channels are mechanosensitive, we speculate they might enable the NPE to respond to stimuli such as mechanical distortion associated with volume homeostasis during fluid transfer across the ciliary epithelium or changes in intraocular pressure.

Damage to lens fiber cells causes TRPV4-dependent Src family kinase activation in the epithelium

The bulk of the lens consists of tightly packed fiber cells. Because mature lens fibers lack mitochondria and other organelles, lens homeostasis relies on a monolayer of epithelial cells at the anterior surface. The detection of various signaling pathways in lens epithelial cells suggests they respond to stimuli that influence lens function. Focusing on Src Family Kinases (SFKs) and Transient Receptor Potential Vanilloid 4 (TRPV4), we tested whether the epithelium can sense and respond to an event that occurs in fiber mass. The pig lens was subjected to localized freeze-thaw (FT) damage to fibers at posterior pole then the lens was incubated for 1-10 min in Krebs solution at 37 °C. Transient SFK activation in the epithelium was detectable at 1 min. Using a western blot approach, the ion channel TRPV4 was detected in the epithelium but was sparse or absent in fiber cells. Even though TRPV4 expression appears low at the actual site of FT damage to the fibers, SFK activation in the epithelium was suppressed in lenses subjected to FT damage then incubated with the TRPV4 antagonist HC067047 (10 μM). Na,K-ATPase activity was examined because previous studies report changes of Na,K-ATPase activity associated with SFK activation. Na,K-ATPase activity doubled in the epithelium removed from FT-damaged lenses and the response was prevented by HC067047 or the SFK inhibitor PP2 (10 μM). Similar changes were observed in response to fiber damage caused by injection of 5 μl hyperosmotic NaCl or mannitol solution beneath the surface of the posterior pole. The findings point to a TRPV4-dependent mechanism that enables the epithelial cells to detect remote damage in the fiber mass and respond within minutes by activating SFK and increasing Na,K-ATPase activity. Because TRPV4 channels are mechanosensitive, we speculate they may be stimulated by swelling of the lens structure caused by damage to the fibers. Increased Na,K-ATPase activity gives the lens greater capacity to control ion concentrations in the fiber mass and the Na,K-ATPase response may reflect the critical contribution of the epithelium to lens ion homeostasis.

A renal-like organic anion transport system in the ciliary epithelium of the bovine and human eye

The purpose of this study was to determine the direction of organic anion (OA) transport across the ciliary body and the transport proteins that may contribute. Transport of several OAs across the bovine ciliary body was examined using ciliary body sections mounted in Ussing chambers and a perfused eye preparation. Microarray, reverse-transcription polymerase chain reaction (RT-PCR), immunoblotting, and immunohistochemistry were used to examine OA transporter expression in human ocular tissues. Microarray analysis showed that many OA transporters common to other barrier epithelia are expressed in ocular tissues. mRNA (RT-PCR) and protein (immunoblotting) for OAT1, OAT3, NaDC3, and MRP4 were detected in extracts of the human ciliary body from several donors. OAT1 and OAT3 localized to basolateral membranes of nonpigmented epithelial cells and MRP4 to basolateral membranes of pigmented cells in the human eye. Para-aminohippurate (PAH) and estrone-3-sulfate transport across the bovine ciliary body in the Ussing chambers was greater in the aqueous humor-to-blood direction than in the blood-to-aqueous humor direction, and active. There was little net directional movement of cidofovir. Probenecid (0.1 mM) or novobiocin (0.1 mM) added to the aqueous humor side of the tissue, or MK571 (5-(3-(2-(7-chloroquinolin-2-yl)ethenyl)phenyl)-8-dimethylcarbamyl-4,6-dithiaoctanoic acid; 0.1 mM) added to the blood side significantly reduced net active PAH transport. The rate of 6-carboxyfluorescein elimination from the aqueous humor of the perfused eye was reduced 80% when novobiocin (0.1 mM) was present in the aqueous humor. These data indicate that the ciliary body expresses a variety of OA transporters, including those common to the kidney. They are likely involved in clearing potentially harmful endobiotic and xenobiotic OAs from the eye.

Calcium entry via connexin hemichannels in lens epithelium

Exposure to hyposmotic solution causes release of ATP from lens cells via hemichannels. Because hemichannel opening feasibly could swamp the cells with calcium, we carried out studies to measure the magnitude of the increase in cytoplasmic calcium concentration caused by hemichannel opening. In studies on porcine lens epithelial cells in primary culture, propidium iodide (PI) uptake was measured as an index of hemichannel opening. PI uptake was increased significantly in cells exposed to hyposmotic solution. The PI increase under hyposmotic conditions was suppressed by GAP 27, a connexin inhibitor peptide. In studies on cells loaded with Fura-2, continuous exposure to hyposmotic solution caused a cytoplasmic calcium concentration increase that peaked within ∼30 s then remained elevated at or below the peak response for more than 60 min. The peak calcium concentration was 186 ± 2.3 nM compared to a baseline value of 98.0 ± 1.4 nM. The calcium concentration increased a lot further in cells exposed to A23187 (2.5 μM) or the sodium-calcium exchange inhibitor SN-6 (10 μM) added after the onset of the calcium rise in hyposmotic solution. The cytoplasmic calcium increase in hyposmotic solution was abolished by GAP 27. Calcium returned to baseline in cells exposed to hyposmotic solution then treated with GAP 27 starting 2 min after the onset of the calcium rise. The calcium increase in hyposmotic solution did not occur when calcium was eliminated from the bathing medium. The responses to hyposmotic and hyperosmotic stress were different. There was no detectable increase in calcium or PI entry in cells exposed to hyperosmotic solution (500mOsm). In summary, GAP 27-sensitive accumulation of PI by cultured lens epithelium points to connexin hemichannel opening and associated calcium entry. Even though connexins form channels with a large carrying capacity, calcium entry does not increase the cytoplasmic calcium concentration beyond a tolerable physiological range.

Purines in the eye: recent evidence for the physiological and pathological role of purines in the RPE, retinal neurons, astrocytes, Müller cells, lens, trabecular meshwork, cornea and lacrimal gland

This review highlights recent findings that describ how purines modulate the physiological and pathophysiological responses of ocular tissues. For example, in lacrimal glands the cross-talk between P2X7 receptors and both M3 muscarinic receptors and α1D-adrenergic receptors can influence tear secretion. In the cornea, purines lead to post-translational modification of EGFR and structural proteins that participate in wound repair in the epithelium and influence the expression of matrix proteins in the stroma. Purines act at receptors on both the trabecular meshwork and ciliary epithelium to modulate intraocular pressure (IOP); ATP-release pathways of inflow and outflow cells differ, possibly permitting differential modulation of adenosine delivery. Modulators of trabecular meshwork cell ATP release include cell volume, stretch, extracellular Ca(2+) concentration, oxidation state, actin remodeling and possibly endogenous cardiotonic steroids. In the lens, osmotic stress leads to ATP release following TRPV4 activation upstream of hemichannel opening. In the anterior eye, diadenosine polyphosphates such as Ap4A act at P2 receptors to modulate the rate and composition of tear secretion, impact corneal wound healing and lower IOP. The Gq11-coupled P2Y1-receptor contributes to volume control in Müller cells and thus the retina. P2X receptors are expressed in neurons in the inner and outer retina and contribute to visual processing as well as the demise of retinal ganglion cells. In RPE cells, the balance between extracellular ATP and adenosine may modulate lysosomal pH and the rate of lipofuscin formation. In optic nerve head astrocytes, mechanosensitive ATP release via pannexin hemichannels, coupled with stretch-dependent upregulation of pannexins, provides a mechanism for ATP signaling in chronic glaucoma. With so many receptors linked to divergent functions throughout the eye, ensuring the transmitters remain local and stimulation is restricted to the intended target may be a key issue in understanding how physiological signaling becomes pathological in ocular disease.

Mechanosensitive release of adenosine 5'-triphosphate through pannexin channels and mechanosensitive upregulation of pannexin channels in optic nerve head astrocytes: a mechanism for purinergic involvement in chronic strain

As adenosine 5'-triphosphate (ATP) released from astrocytes can modulate many neural signaling systems, the triggers and pathways for this ATP release are important. Here, the ability of mechanical strain to trigger ATP release through pannexin channels and the effects of sustained strain on pannexin expression were examined in rat optic nerve head astrocytes. Astrocytes released ATP when subjected to 5% of equibiaxial strain or to hypotonic swelling. Although astrocytes expressed mRNA for pannexins 1-3, connexin 43, and VNUT, pharmacological analysis suggested a predominant role for pannexins in mechanosensitive ATP release, with Rho kinase contribution. Astrocytes from panx1(-/-) mice had reduced baseline and stimulated levels of extracellular ATP, confirming the role for pannexins. Swelling astrocytes triggered a regulatory volume decrease that was inhibited by apyrase or probenecid. The swelling-induced rise in calcium was inhibited by P2X7 receptor antagonists A438079 and AZ10606120, in addition to apyrase and carbenoxolone. Extended stretch of astrocytes in vitro upregulated expression of panx1 and panx2 mRNA. A similar upregulation was observed in vivo in optic nerve head tissue from the Tg-MYOC(Y437H) mouse model of chronic glaucoma; genes for panx1, panx2, and panx3 were increased, whereas immunohistochemistry confirmed increased expression of pannexin 1 protein. In summary, astrocytes released ATP in response to mechanical strain, with pannexin 1 the predominant efflux pathway. Sustained strain upregulated pannexins in vitro and in vivo. Together, these findings provide a mechanism by which extracellular ATP remains elevated under chronic mechanical strain, as found in the optic nerve head of patients with glaucoma.

Nitric oxide regulation of Na, K-ATPase activity in ocular ciliary epithelium involves Src family kinase

The nitric oxide (NO) donor sodium nitroprusside (SNP) is known to reduce aqueous humor (AH) secretion in the isolated porcine eye. Previously, SNP was found to inhibit Na,K-ATPase activity in nonpigmented ciliary epithelium (NPE), AH-secreting cells, through a cGMP/protein kinase G (PKG)-mediated pathway. Here we show Src family kinase (SFK) activation in the Na,K-ATPase activity response to SNP. Ouabain-sensitive (86) Rb uptake was reduced by >35% in cultured NPE cells exposed to SNP (100 µM) or exogenously added cGMP (8-Br-cGMP) (100 µM) and the SFK inhibitor PP2 (10 µM) prevented the response. Ouabain-sensitive ATP hydrolysis was reduced by ~40% in samples detected in material obtained from SNP- and 8-Br-cGMP-treated cells following homogenization, pointing to an intrinsic change of Na,K-ATPase activity. Tyrosine-10 phosphorylation of Na,K-ATPase α1 subunit was detected in SNP and L-arginine-treated cells and the response prevented by PP2. SNP elicited an increase in cell cGMP. Cells exposed to 8-Br-cGMP displayed SFK activation (phosphorylation) and inhibition of both ouabain-sensitive (86) Rb uptake and Na,K-ATPase activity that was prevented by PP2. SFK activation, which also occurred in SNP-treated cells, was suppressed by inhibitors of soluble guanylate cyclase (ODQ; 10 µM) and PKG (KT5823; 1 µM). SNP and 8-Br-cGMP also increased phosphorylation of ERK1/2 and p38 MAPK and the response prevented by PP2. However, U0126 did not prevent SNP or 8-Br-cGMP-induced inhibition of Na,K-ATPase activity. Taken together, the results suggest that NO activates guanylate cyclase to cause a rise in cGMP and subsequent PKG-dependent SFK activation. Inhibition of Na,K-ATPase activity depends on SFK activation.

Nonpigmented ciliary epithelial cells respond to acetazolamide by a soluble adenylyl cyclase mechanism

PURPOSE

The nonpigmented ciliary epithelium (NPE) is rich in soluble adenylyl cyclase (sAC), a proposed cytoplasmic bicarbonate sensor. Here, we examine the contribution of sAC to an increase in cyclic AMP (cAMP) and changes in a key ion transporter, H(+)-ATPase, in NPE exposed to acetazolamide, a carbonic anhydrase inhibitor (CAI).

METHODS

Cyclic AMP was measured by radioimmunoassay in primary cultured porcine NPE. The pH-sensitive dye BCECF was used to examine cytoplasmic pH regulation. Subcellular protein translocation was examined by Western blot.

RESULTS

A transient cAMP increase, detectable within minutes of acetazolamide treatment, was prevented by KH7, a specific sAC inhibitor. Following 10-minute exposure to acetazolamide, the abundance of H(+)-ATPase B1 subunit and sAC was doubled in a plasma membrane-rich fraction, suggesting subcellular translocation. Similar evidence of H(+)-ATPase translocation was observed in NPE exposed to 8-Bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP). Consistent with increased capacity for proton export, acetazolamide increased the rate of pH recovery from acidification. KH7 and bafilomycin A1, an inhibitor of H(+)-ATPase, both prevented the stimulatory effect of acetazolamide on pH recovery. In a parallel study, H(+)-ATPase abundance was found to be higher in the plasma membrane of HEK293 cells that overexpress sAC compared to the normal HEK293 cells. HEK cells that overexpress sAC and had higher H(+)-ATPase abundance displayed a faster rate of pH recovery and greater sensitivity to KH7.

CONCLUSIONS

Acetazolamide increases cAMP in a response that involves activation of sAC. Subcellular translocation of H(+)-ATPase and an increase in the capacity for proton export by acetazolamide-treated NPE cells is a cAMP-dependent response.

DIDS inhibits Na-K-ATPase activity in porcine nonpigmented ciliary epithelial cells by a Src family kinase-dependent mechanism

The anion transport inhibitor DIDS is known to reduce aqueous humor secretion but questions remain about anion dependence of the effect. In some tissues, DIDS is reported to cause Na-K-ATPase inhibition. Here, we report on the ability of DIDS to inhibit Na-K-ATPase activity in nonpigmented ciliary epithelium (NPE) and investigate the underlying mechanism. Porcine NPE cells were cultured to confluence on permeable supports, treated with drugs added to both sides of the membrane, and then used for (86)Rb uptake measurements or homogenized to measure Na-K-ATPase activity or to detect protein phosphorylation. DIDS inhibited ouabain-sensitive (86)Rb uptake, activated Src family kinase (SFK), and caused a reduction of Na-K-ATPase activity. PP2, an SFK inhibitor, prevented the DIDS responses. In BCECF-loaded NPE, DIDS was found to reduce cytoplasmic pH (pHi). PP2-sensitive Na-K-ATPase activity inhibition, (86)Rb uptake suppression, and SFK activation were observed when a similar reduction of pHi was imposed by low-pH medium or an ammonium chloride withdrawal maneuver. PP2 and the ERK inhibitor U0126 prevented robust ERK1/2 activation in cells exposed to DIDS or subjected to pHi reduction, but U0126 did not prevent SFK activation or the Na-K-ATPase activity response. The evidence points to an inhibitory influence of DIDS on NPE Na-K-ATPase activity by a mechanism that hinges on SFK activation associated with a reduction of cytoplasmic pH.

Vacuolar ATPase driven potassium transport in highly metastatic breast cancer cells

Breast cancer is the second leading cause of death in women and thus has received a great deal of attention by researchers. Recent studies suggested decreased occurrence of cancer in patients treated with cardiac glycosides (CGs) for heart conditions. Because CGs induce their cellular effects via the Na(+), K(+) ATPase (Na-K), we treated four breast cancer cell lines (MCF-7, T47D, MDA-MB453, and MDA-MB231) and a non-cancerous breast ductal epithelial cell line (MCF-10A) with ouabain, a well-characterized CG, and measured cell proliferation by measuring bromodeoxyuridine incorporation. Ouabain (1μM) decreased cell proliferation in all cell lines studied except MDA-MB453 cells. Western blot of Na-K α and β subunits showed α1, α3, and β1 expression in all cell lines except MDA-MB453 cells where Na-K protein and mRNA were absent. Potassium uptake, measured as rubidium ((86)Rb) flux, and intracellular potassium were both significantly higher in MDA-MB453 cells compared to MCF-10A cells. RT-qPCR suggested a 7 fold increase in voltage-gated potassium channel (KCNQ2) expression in MDA-MB453 cells compared to MCF-10A cells. Inhibition of KCNQ2 prevented cell growth and (86)Rb uptake in MDA-MB453 cells but not in MCF-10A cells. All cancer cells had significantly higher vacuolar H-ATPase (V-ATPase) activity than MCF-10A cells. Inhibition of V-ATPase decreased (86)Rb uptake and intracellular potassium in MDA-MB453 cells but not in MCF-10A cells. The findings point to the absence of Na-K, high hERG and KCNQ2 expression, elevated V-ATPase activity and sensitivity to V-ATPase inhibitors in MDA-MB453. We conclude that cancer cells exhibit fundamentally different metabolic pathways for maintenance of intracellular ion homeostasis.

TRPV4 in porcine lens epithelium regulates hemichannel-mediated ATP release and Na-K-ATPase activity

In several tissues, transient receptor potential vanilloid 4 (TRPV4) channels are involved in the response to hyposmotic challenge. Here we report TRPV4 protein in porcine lens epithelium and show that TRPV4 activation is an important step in the response of the lens to hyposmotic stress. Hyposmotic solution (200 mosM) elicited ATP release from intact lenses and TRPV4 antagonists HC 067047 and RN 1734 prevented the release. In isosmotic solution, the TRPV4 agonist GSK1016790A (GSK) elicited ATP release. When propidium iodide (PI) (MW 668) was present in the bathing medium, GSK and hyposmotic solution both increased PI entry into the epithelium of intact lenses. Increased PI uptake and ATP release in response to GSK and hyposmotic solution were abolished by a mixture of agents that block connexin and pannexin hemichannels, 18α-glycyrrhetinic acid and probenecid. Increased Na-K-ATPase activity occurred in the epithelium of lenses exposed to GSK and 18α-glycyrrhetinic acid + probenecid prevented the response. Hyposmotic solution caused activation of Src family kinase and increased Na-K-ATPase activity in the lens epithelium and TRPV4 antagonists prevented the response. Ionomycin, which is known to increase cytoplasmic calcium, elicited ATP release, the magnitude of which was no greater when lenses were exposed simultaneously to ionomycin and hyposmotic solution. Ionomycin-induced ATP release was significantly reduced in calcium-free medium. TRPV4-mediated calcium entry was examined in Fura-2-loaded cultured lens epithelium. Hyposmotic solution and GSK both increased cytoplasmic calcium that was prevented by TRPV4 antagonists. The cytoplasmic calcium rise in response to hyposmotic solution or GSK was abolished when calcium was removed from the bathing solution. The findings are consistent with hyposmotic shock-induced TRPV4 channel activation which triggers hemichannel-mediated ATP release. The results point to TRPV4-mediated calcium entry that causes a cytoplasmic calcium increase which is an essential early step in the mechanism used by the lens to sense and respond to hyposmotic stress.

Hyposmotic stress causes ATP release and stimulates Na,K-ATPase activity in porcine lens

Purinergic receptors in lens epithelium suggest lens function can be altered by chemical signals from aqueous humor or the lens itself. Here we show release of ATP by intact porcine lenses exposed to hyposmotic solution (200 mOsm). 18α-glycyrrhetinic acid (AGA) added together with probenecid eliminated the ATP increase. N-ethylmaleimide (200 µM), an exocytotic inhibitor, had no significant effect on ATP increase. Lenses exposed to hyposmotic solution displayed a ~400% increase of propidium iodide (PI) entry into the epithelium. The increased ability of PI (MW 668) to enter the epithelium suggests possible opening of connexin and/or pannexin hemichannels. This is consistent with detection of connexin 43, connexin 50, and pannexin 1 in the epithelium and the ability of AGA + probenecid to prevent ATP release. Na,K-ATPase activity doubled in the epithelium of lenses exposed to hyposmotic solution. The increase of Na,K-ATPase activity did not occur when apyrase was used to prevent extracellular ATP accumulation or when AGA + probenecid prevented ATP release. The increase of Na,K-ATPase activity was inhibited by the purinergic P2 antagonist reactive blue-2 and pertussis toxin, a G-protein inhibitor, but not by the P2X antagonist PPADS. Hyposmotic solution activated Src family kinase (SFK) in the epithelium, judged by Western blot. The SFK inhibitor PP2 abolished both SFK activation and the Na,K-ATPase activity increase. In summary, hyposmotic shock-induced ATP release is sufficient to activate a purinergic receptor- and SFK-dependent mechanism that stimulates Na,K-ATPase activity. The responses might signify an autoregulatory loop initiated by mechanical stress or osmotic swelling.

The effect of endothelin-1 on Src-family tyrosine kinases and Na,K-ATPase activity in porcine lens epithelium

Previous studies show Src family kinase (SFK) activation is involved in a response that stimulates Na,K-ATPase. Here, we tested whether SFK activation is involved in the Na,K-ATPase response to endothelin-1 (ET-1). Intact porcine lenses were exposed to 100 nM ET-1 for 5-30 min. Then, the epithelium was removed and used for Na,K-ATPase activity measurement and Western blot analysis of SFK activation. Na,K-ATPase activity was reduced by ∼30% in lenses exposed to ET-1 for 15 min. The response was abolished by the SFK inhibitor PP2 or the ET receptor antagonist, PD145065. Activation of a ∼61 kDa SFK was evident from an increase in Y416 phosphorylation, which reached a maximum at 15 min ET-1 treatment, and a decrease in Y527 phosphorylation. PP2 prevented SFK activation. Since Fyn, Src, Hck, and Yes may contribute to the observed 61 kDa band, these SFKs were isolated by immunoprecipitation and analyzed. Based on Y416 phosphorylation, ET-1 appeared to activate Fyn, while Src and Hck were inhibited and Yes was unaltered. ET-1 requires SFK activation to cause Na,K-ATPase inhibition. ET-1 elicits a different pattern of SFK activation from that reported earlier for purinergic agonists that stimulate Na,K-ATPase activity and activate Src. In the ET-1 response Src is inhibited and Fyn is activated. The findings suggest SFK phosphorylation is involved in a regulatory mechanism for Na,K-ATPase. Knowing this may help us understand drug actions on Na,K-ATPase. Faulty regulation of Na,K-ATPase in the lens could contribute to cataract formation since an abnormal sodium content is associated with lens opacification.

Genetic variation of the alpha subunit of the epithelial Na+ channel influences exhaled Na+ in healthy humans

Epithelial Na(+) channels (ENaC) are located in alveolar cells and are important in β(2)-adrenergic receptor-mediated lung fluid clearance through the removal of Na(+) from the alveolar airspace. Previous work has demonstrated that genetic variation of the alpha subunit of ENaC at amino acid 663 is important in channel function: cells with the genotype resulting in alanine at amino acid 663 (A663) demonstrate attenuated function when compared to genotypes with at least one allele encoding threonine (T663, AT/TT). We sought to determine the influence of genetic variation at position 663 of ENaC on exhaled Na(+) in healthy humans. Exhaled Na(+) was measured in 18 AA and 13 AT/TT subjects (age=27±8 years vs. 30±10 years; ht.=174±12 cm vs. 171±10 cm; wt.=68±12 kg vs. 73±14 kg; BMI=22±3 kg/m(2) vs. 25±4 kg/m(2), mean±SD, for AA and AT/TT, respectively). Measurements were made at baseline and at 30, 60 and 90 min following the administration of a nebulized β(2)-agonist (albuterol sulfate, 2.5 mg diluted in 3 ml normal saline). The AA group had a higher baseline level of exhaled Na(+) and a greater response to β(2)-agonist stimulation (baseline=3.1±1.8 mmol/l vs. 2.3±1.5 mmol/l; 30 min-post=2.1±0.7 mmol/l vs. 2.2±0.8 mmol/l; 60 min-post=2.0±0.5 mmol/l vs. 2.3±1.0 mmol/l; 90 min-post=1.8±0.8 mmol/l vs. 2.6±1.5 mmol/l, mean±SD, for AA and AT/TT, respectively, p<0.05). The results are consistent with the notion that genetic variation of ENaC influences β(2)-adrenergic receptor stimulated Na(+) clearance in the lungs, as there was a significant reduction in exhaled Na(+) over time in the AA group.

The Na+/H+ exchanger controls deoxycholic acid-induced apoptosis by a H+-activated, Na+-dependent ionic shift in esophageal cells

Apoptosis resistance is a hallmark of cancer cells. Typically, bile acids induce apoptosis. However during gastrointestinal (GI) tumorigenesis the cancer cells develop resistance to bile acid-induced cell death. To understand how bile acids induce apoptosis resistance we first need to identify the molecular pathways that initiate apoptosis in response to bile acid exposure. In this study we examined the mechanism of deoxycholic acid (DCA)-induced apoptosis, specifically the role of Na⁺/H⁺ exchanger (NHE) and Na⁺ influx in esophageal cells. In vitro studies revealed that the exposure of esophageal cells (JH-EsoAd1, CP-A) to DCA (0.2 mM -0.5 mM) caused lysosomal membrane perturbation and transient cytoplasmic acidification. Fluorescence microscopy in conjunction with atomic absorption spectrophotometry demonstrated that this effect on lysosomes correlated with influx of Na⁺, subsequent loss of intracellular K⁺, an increase of Ca²⁺ and apoptosis. However, ethylisopropyl-amiloride (EIPA), a selective inhibitor of NHE, prevented Na⁺, K⁺ and Ca²⁺ changes and caspase 3/7 activation induced by DCA. Ouabain and amphotericin B, two drugs that increase intracellular Na⁺ levels, induced similar changes as DCA (ion imbalance, caspase3/7 activation). On the contrary, DCA-induced cell death was inhibited by medium with low a Na⁺ concentrations. In the same experiments, we exposed rat ileum ex-vivo to DCA with or without EIPA. Severe tissue damage and caspase-3 activation was observed after DCA treatment, but EIPA almost fully prevented this response. In summary, NHE-mediated Na⁺ influx is a critical step leading to DCA-induced apoptosis. Cells tolerate acidification but evade DCA-induced apoptosis if NHE is inhibited. Our data suggests that suppression of NHE by endogenous or exogenous inhibitors may lead to apoptosis resistance during GI tumorigenesis.

The Physiology undergraduate major in the University of Arizona College of Medicine: past, present, and future

The American Physiological Society (APS) and APS Council encourage the teaching of physiology at the undergraduate, graduate, and medical school levels to support the continued prominence of this area of science. One area identified by the APS Council that is of particular importance for the development of future physiologists (the "physiology pipeline") is the teaching of physiology and physiology-related topics at the undergraduate level. In this article, we describe the historical development and implementation of an undergraduate program offered through the Department of Physiology, a basic science department in the College of Medicine at the University of Arizona, culminating in a Bachelor of Science in Health Sciences degree with a major in Physiology. Moreover, we discuss the current Physiology curriculum offered at our institution and explain how this program prepares our students for successful entry into a variety of postbaccalaureate professional programs, including medical school and numerous other programs in health professions, and in graduate study in the Masters and Doctoral programs in biomedical sciences. Finally, we cover the considerable challenges that we have faced, and continue to face, in developing and sustaining a successful physiology undergraduate major in a college of medicine. We hope that the information provided on the Physiology major offered by the Department of Physiology in the College of Medicine at the University of Arizona will be helpful for individuals at other institutions who may be contemplating the development and implementation of an undergraduate program in Physiology.

New therapeutic and biomarker discovery for peripheral diabetic neuropathy: PARP inhibitor, nitrotyrosine, and tumor necrosis factor-{alpha}

This study evaluated poly(ADP-ribose) polymerase (PARP) inhibition as a new therapeutic approach for peripheral diabetic neuropathy using clinically relevant animal model and endpoints, and nitrotyrosine (NT), TNF-alpha, and nitrite/nitrate as potential biomarkers of the disease. Control and streptozotocin-diabetic rats were maintained with or without treatment with orally active PARP inhibitor 10-(4-methyl-piperazin-1-ylmethyl)-2H-7-oxa-1,2-diaza-benzo[de]anthracen-3-one (GPI-15,427), 30 mg kg(-1) d(-1), for 10 wk after first 2 wk without treatment. Therapeutic efficacy was evaluated by poly(ADP-ribosyl)ated protein expression (Western blot analysis), motor and sensory nerve conduction velocities, and tibial nerve morphometry. Sciatic nerve and spinal cord NT, TNF-alpha, and nitrite/nitrate concentrations were measured by ELISA. NT localization in peripheral nervous system was evaluated by double-label fluorescent immunohistochemistry. A PARP inhibitor treatment counteracted diabetes-induced motor and sensory nerve conduction slowing, axonal atrophy of large myelinated fibers, and increase in sciatic nerve and spinal cord NT and TNF-alpha concentrations. Sciatic nerve NT and TNF-alpha concentrations inversely correlated with motor and sensory nerve conduction velocities and myelin thickness, whereas nitrite/nitrate concentrations were indistinguishable between control and diabetic groups. NT accumulation was identified in endothelial and Schwann cells of the peripheral nerve, neurons, astrocytes, and oligodendrocytes of the spinal cord, and neurons and glial cells of the dorsal root ganglia. The findings identify PARP as a compelling drug target for prevention and treatment of both functional and structural manifestations of peripheral diabetic neuropathy and provide rationale for detailed evaluation of NT and TNF-alpha as potential biomarkers of its presence, severity, and progression.

Ouabain stimulates Na-K-ATPase through a sodium/hydrogen exchanger-1 (NHE-1)-dependent mechanism in human kidney proximal tubule cells

Recent investigations demonstrate increased Na/H exchanger-1 (NHE-1) activity and plasma levels of ouabain-like factor in spontaneously hypertensive rats. At nanomolar concentrations, ouabain increases Na-K-ATPase activity, induces cell proliferation, and activates complex signaling cascades. We hypothesize that the activity of NHE-1 and Na-K-ATPase are interdependent. To test whether treatment with picomolar ouabain regulates Na-K-ATPase through an NHE-1-dependent mechanism, we examined the role of NHE-1 in ouabain-mediated stimulation of Na-K-ATPase in kidney proximal tubule cell lines [opossum kidney (OK), HK-2, HKC-5, and HKC-11] and rat kidney basolateral membranes. Ouabain stimulated Na-K-ATPase activity and tyrosine phosphorylation in cells that express NHE-1 (OK, HKC-5, and HKC-11) but not in HK-2 cells that express very low levels of NHE-1. Inhibition of NHE-1 with 5 microM EIPA, a NHE-1-specific inhibitor, prevented ouabain-mediated stimulation of (86)Rb uptake and Na-K-ATPase phosphorylation in OK, HKC-5, and HKC-11 cells. Expression of wild-type NHE-1 in HK2 cells restored regulation of Na-K-ATPase by picomolar ouabain. Treatment with picomolar ouabain increased membrane expression of Na-K-ATPase and enhanced NHE-1-Na-K-ATPase alpha1-subunit association. Treatment with ouabain (1 microg x kg body wt(-1) x day(-1)) increased Na-K-ATPase activity, expression, phosphorylation, and association with NHE-1 increased in rat kidney cortical basolateral membranes. Eight days' treatment with ouabain (1 microg x kg body wt(-1) x day(-1)) resulted in increased blood pressure in these rats. These results suggest that the association of NHE-1 with Na-K-ATPase is critical for ouabain-mediated regulation of Na-K-ATPase and that these effects may play a role in cardioglycoside-stimulated hypertension.

Hydrostatic pressure-induced release of stored calcium in cultured rat optic nerve head astrocytes

PURPOSE

Elevated intraocular pressure is associated with glaucomatous optic nerve damage. Other investigators have shown functional changes in optic nerve head astrocytes subjected to elevated hydrostatic pressure (HP) for 1 to 5 days. Recently, the authors reported ERK1/2, p90(RSK) and NHE1 phosphorylation after 2 hours. Here they examine calcium responses at the onset of HP to determine what precedes ERK1/2 phosphorylation.

METHODS

Cytoplasmic calcium concentration ([Ca(2+)](i)) was measured in cultured rat optic nerve astrocytes loaded with fura-2. The cells were placed in a closed imaging chamber and subjected to an HP increase of 15 mm Hg. Protein phosphorylation was detected by Western blot analysis.

RESULTS

The increase of HP caused an immediate slow increase in [Ca(2+)](i). The response persisted in calcium-free solution and when nickel chloride (4 mM) was added to suppress channel-mediated calcium entry. Previous depletion of the ER calcium stores by cyclopiazonic acid abolished the HP-induced calcium level increase. The HP-induced increase persisted in cells exposed to xestospongin C, an inhibitor of IP3R-mediated calcium release. In contrast, ryanodine receptor (RyR) antagonist ruthenium red (10 microM) or dantrolene (25 microM) inhibited the HP-induced calcium increase. The HP-induced calcium increase was abolished when ryanodine-sensitive calcium stores were pre-depleted with caffeine (3 mM). HP caused ERK1/2 phosphorylation. The magnitude of the ERK1/2 phosphorylation response was reduced by ruthenium red and dantrolene.

CONCLUSIONS

Increasing HP causes calcium release from a ryanodine-sensitive cytoplasmic store and subsequent ERK1/2 activation. Calcium store release appears to be a required early step in the initial astrocyte response to an HP increase.

Responses of sodium-hydrogen exchange to nitric oxide in porcine cultured nonpigmented ciliary epithelium

PURPOSE

To better understand how nitric oxide (NO) alters the function of the nonpigmented ciliary epithelium (NPE), studies were performed to determine the influence of NO on sodium-hydrogen exchanger (NHE) activity.

METHODS

Cytoplasmic pH (pH(i)) was measured in cultured porcine NPE loaded with BCECF (2',7'-bis(2-carboxyl)-5(6)-carboxyfluorescein-acetoxyethyl ester). Na-H exchanger (NHE) was examined by immunolocalization.

RESULTS

In cells acidified by 5 minutes of exposure to 20 mM ammonium chloride, pH(i) recovery was partially inhibited by sodium nitroprusside (SNP), an NO donor, and l-arginine, the endogenous substrate for NO synthase. SNP and dimethyl amiloride (DMA), an NHE inhibitor, inhibited pH(i) recovery to a similar degree. In bicarbonate-free buffer SNP+DMA elicited no additional change in pH(i) recovery beyond that elicited by DMA alone. This suggests that SNP causes NHE inhibition. the SNP's effect on pH(i) recovery was mimicked by 8-pCPT-cGMP but suppressed by ODQ and H-8. Ouabain alone reduced pH(i) recovery, but SNP+ouabain caused significant further reduction. Immunolocalization studies revealed NHE1 and -4 in native and cultured NPE.

CONCLUSIONS

NHE1 and -4 are expressed at the NPE basolateral margin. The findings suggest the NHE is inhibited by NO which acts via a cGMP and protein kinase G signaling pathway. The NHE response does not appear to be the consequence of NO-induced Na,K-ATPase inhibition. Because NO synthases are expressed in porcine NPE, NO could act as an autocrine regulator of NHE activity. Although NHE inhibitors are known to lower intraocular pressure (IOP), further studies are needed to understand whether changes in NHE activity contribute to the IOP-lowering effect of NO donors.

Elevated hydrostatic pressure activates sodium/hydrogen exchanger-1 in rat optic nerve head astrocytes

Optic nerve head astrocytes become abnormal in eyes that have elevated intraocular pressure, and cultured astrocytes display altered protein expression after being subjected for > or = 1 days to elevated hydrostatic pressure. Here we show that 2-h elevated hydrostatic pressure (15 or 30 mmHg) causes phosphorylation of ERK1/2, ribosomal S6 protein kinase (p90(RSK)), and Na/H exchanger (NHE)1 in cultured rat optic nerve head astrocytes as judged by Western blot analysis. The MEK/ERK inhibitor U0126 abolished phosphorylation of NHE1 and p90(RSK) as well as ERK1/2. To examine NHE1 activity, cytoplasmic pH (pH(i)) was measured with BCECF and, in some experiments, cells were acidified by 5-min exposure to 20 mM ammonium chloride. Although baseline pH(i) was unaltered, the rate of pH(i) recovery from acidification was fourfold higher in pressure-treated astrocytes. In the presence of either U0126 or dimethylamiloride (DMA), an NHE inhibitor, hydrostatic pressure did not change the rate of pH(i) recovery. The findings are consistent with NHE1 activation due to phosphorylation of ERK1/2, p90(RSK), and NHE1 that occurs in response to hydrostatic pressure. These responses may precede long-term changes of protein expression known to occur in pressure-stressed astrocytes.

Localization of multidrug resistance-associated protein 2 in the nonpigmented ciliary epithelium of the eye

The nonpigmented epithelium (NPE) of the ciliary body represents an important component of the blood-aqueous barrier of the eye. Many therapeutic drugs penetrate poorly across the NPE into the aqueous humor of the eye interior. Several of these therapeutic drugs, such as methotrexate, vincristine, and etoposide, are substrates of the multidrug resistance-associated protein 2 (MRP2). Abundant MRP2 protein was detected by Western blot in homogenates of human ciliary body and freshly dissected porcine NPE. In cultured porcine NPE, the intracellular accumulation of the MRP2 substrates calcein (1.8-fold), 5-(and-6)-carboxy-2',7'-dichlorofluorescein (22.1-fold), and doxorubicin (1.9-fold) was significantly increased in the presence of 50 microM MK571 ((E)-3-[[[3-[2-(7-chloro-2-quinolinyl)-ethenyl]phenyl]-[[3-dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid), an MRP inhibitor. In addition, the intracellular accumulation of the MRP2 substrate glutathione methylfluorescein was increased by 50 microM MK571 (4.3-fold), 500 microM indomethacin (2.6-fold), and 50 microM cyclosporin A (2.1-fold) but not by 500 microM sulfinpyrazone. These data are consistent with MRP2-mediated transport activity in cultured NPE, and MRP2 mRNA (reverse transcriptase-polymerase chain reaction) and protein (Western blot) were detected in the cultured cells. Immunolocalization studies in native human and porcine eyes showed MRP2 protein at the apical interface of the NPE and pigmented cell layers. Close examination of MRP2 immunoreactivity supported the conclusion that MRP2 is localized in the apical membrane of the NPE. MRP2 at the apical membrane of NPE cells may be involved in protecting intraocular tissues from exposure to potentially harmful toxins.