The influence of sodium–calcium exchange inhibitors on rabbit lens ion balance and transparency

Expression of potassium-dependent sodium-calcium exchanger in the murine lens

Loss of intracellular calcium homeostasis may contribute to the opacification of lens tissue during cortical cataract formation. In healthy lenses, the concentration of intracellular calcium is maintained at levels far below electrochemical equilibrium but the identity of the calcium extrusion mechanism in lens fiber cells has remained elusive. Previous studies focused on the role of plasma membrane calcium ATPases and sodium-calcium exchangers. Here, we examined the expression of mRNA transcripts encoding potassium-dependent sodium-calcium exchangers (Nckx's, encoded by the Slc24 gene family) in the mouse lens. The most abundant of the five Slc24 family members was Slc24a4 (Nckx4). Notably, Slc24a4 was the only family member with increased expression in fiber cells. Using an antibody raised against recombinant mouse Nckx4, we showed that the protein is expressed strongly in the outer cortical fibers, consistent with results of in situ hybridization experiments and earlier mass spectrometry analysis. To test the role of Nckx4 directly, we generated mice in which Slc24a4 was deleted conditionally in lens tissue. In conditional knockout animals, the level of Nckx4 protein was reduced to background levels without a discernible effect on lens growth or transparency. Thus, despite its relative abundance in the lens, Nckx4 does not appear to have an indispensable role in the maintenance of lens clarity.

Lens gap junctions in growth, differentiation, and homeostasis

The cells of most mammalian organs are connected by groups of cell-to-cell channels called gap junctions. Gap junction channels are made from the connexin (Cx) family of proteins. There are at least 20 isoforms of connexins, and most tissues express more than 1 isoform. The lens is no exception, as it expresses three isoforms: Cx43, Cx46, and Cx50. A common role for all gap junctions, regardless of their Cx composition, is to provide a conduit for ion flow between cells, thus creating a syncytial tissue with regard to intracellular voltage and ion concentrations. Given this rather simple role of gap junctions, a persistent question has been: Why are there so many Cx isoforms and why do tissues express more than one isoform? Recent studies of lens Cx knockout (KO) and knock in (KI) lenses have begun to answer these questions. To understand these roles, one must first understand the physiological requirements of the lens. We therefore first review the development and structure of the lens, its numerous transport systems, how these systems are integrated to generate the lens circulation, the roles of the circulation in lens homeostasis, and finally the roles of lens connexins in growth, development, and the lens circulation.

Absence of SPARC leads to impaired lens circulation

SPARC is a matricellular glycoprotein involved in regulation of extracellular matrix, growth factors, adhesion, and migration. SPARC-null mice have altered basement membranes and develop posterior sub-capsular cataracts with cell swelling and equatorial vacuoles. Exchange of fluid, nutrients, and waste products in the avascular lens is driven by a unique circulating ion current. In the absence of SPARC, increased circulation of fluid, ions, and small molecules led to increased fluorescein distribution in vivo, loss of resting membrane polarization, and altered distribution of small molecules. Microarray analysis of SPARC-null lenses showed changes in gene expression of ion channels and receptors, matrix and adhesion genes, cytoskeleton, immune response genes, and cell signaling molecules. Our results confirm the hypothesis that the regulation of SPARC on cell-capsular matrix interactions can increase the circulation of fluid and ions in the lens, and the phenotype in the SPARC-null mouse lens is the result of multiple intersecting functional pathways.

The mechanisms of calcium homeostasis and signalling in the lens

Excessive Ca(2+) can be detrimental to cells and raised levels of Ca(2+) in human lenses with cortical cataract have been found to play a major role in the opacification process. Ca(2+) homeostasis is therefore, recognised as having fundamental importance in lens pathophysiology. Furthermore, Ca(2+) plays a central role as a second messenger in cell signalling and mechanisms have evolved which give cells exquisite control over intracellular Ca(2+) ([Ca(2+)](i)) via an array of specialised regulatory and signalling proteins. In this review we discuss these mechanisms as they apply to the lens. Ca(2+) levels in human aqueous humour are approximately 1 mM and there is a large, 10,000 fold, inwardly directed gradient across the plasma membrane. In the face of such a large gradient highly efficient mechanisms are needed to maintain low [Ca(2+)](i). The Na(+)/Ca(2+) exchanger (NCX) and plasma membrane Ca(2+)-ATPase (PMCA) actively remove Ca(2+) from the cells, whereas the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) sequesters Ca(2+) in the endoplasmic reticulum (ER) Ca(2+) store. In lens epithelial cells the dominant role is played by the ATPases, whilst in the fibre cells NCX activity appears to be more important. Usually, [Ca(2+)](i) can be increased in a number of ways. Ca(2+) influx through the plasma membrane, for example, is mediated by an array of channels with evidence in the lens for the presence of voltage-operated Ca(2+) channels (VOCCs), receptor-operated Ca(2+) channels (ROCCs) and channels mediating store-operated Ca(2+) entry (SOCE). Ca(2+) signalling is initiated via activation of G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTK) of which the lens expresses a surprisingly diverse array responding to various neurotransmitters, hormones, growth factors, autocoids and proteases. Downstream of plasma membrane receptors are IP(3)-gated channels (IP(3)Rs) and ryanodine receptors (RYRs) located in the ER, which when activated cause a rapid increase in [Ca(2+)](i) and these have also been identified in the lens. Through an appreciation of the diversity and complexity of the mechanisms involved in Ca(2+) homeostasis in normal lens cells we move closer to an understanding of the mechanisms which mediate pathological Ca(2+) overload as occurs in the process of cataract formation.

Ouabain-induced stimulation of sodium-hydrogen exchange in rat optic nerve astrocytes

Sodium-dependent transporters are inhibited indirectly by the Na-K-ATPase inhibitor ouabain. Here we report stimulation of sodium-hydrogen exchange (NHE) in ouabain-treated cells. BCECF was used to measure cytoplasmic pH in cultured rat optic nerve astrocytes. Ammonium chloride was applied to acid load the cells. On removal of ammonium chloride, cytoplasmic pH fell abruptly, then gradually recovered toward baseline. Ouabain (1 microM) did not change cell sodium content, but the rate of pH recovery increased by 68%. Ouabain speeded pH recovery both in the presence and absence of bicarbonate. In bicarbonate-free medium, dimethylamiloride, an NHE inhibitor, eliminated the effect of 1 microM ouabain on pH recovery. Western blot analysis showed an NHE1 immunoreactive band but not NHE2, NHE3, or NHE4. Immunoprecipitation studies showed phosphorylation of NHE1 in cells treated with 1 microM ouabain. Ouabain evoked an increase of cAMP, and the effect of 1 microM ouabain on pH recovery was abolished by H-89, a protein kinase A inhibitor. 8-Bromoadenosine-cAMP increased the pH recovery rate, and this recovery was not further increased by ouabain. Although 1 microM ouabain did not alter cytoplasmic calcium concentration, it stimulated calcium entry after store depletion, a response abolished by 2-APB. Ouabain-induced stimulation of pH recovery was suppressed by inhibitors of capacitative calcium entry, SKF-96365, and 2-APB, as well as the cytoplasmic calcium chelator BAPTA. The cAMP increase in ouabain-treated cells was abolished by BAPTA and 2-APB. Taken together, the results are consistent with increased capacitative calcium entry and subsequent cAMP-PKA-dependent stimulation of NHE1 in ouabain-treated cells.

Primary culture of porcine nonpigmented ciliary epithelium

PURPOSE

Primary culture of nonpigmented ciliary epithelium (NPE) has proved difficult in the past. Here we report development of a method of growing and maintaining primary cultures of NPE from porcine eye. Studies were conducted to confirm that the cultured NPE expressed proteins characteristic of native NPE.

METHODS

Intact rings of NPE were isolated from adult pig eyes. A mixture of hyaluronidase and collagenase was used to detach the cells from the basement membrane and vitreous. Dispersed cells were seeded at high density and grown in DMEM with 20% fetal bovine serum under 5% CO2 and 95% air. Protein expression was examined by immunohistochemistry and immunoblot analysis.

RESULTS

NPE cells were grown in primary culture and maintained up to 10th passage. Analysis of the ciliary body showed three Na, K-ATPase isoforms (alpha 1, alpha 2, alpha 3) and three nitric oxide synthase isoforms (eNOS, nNOS, iNOS) enriched in the NPE layer but weaker or absent in the PE layer. Each of these proteins as well as the tight junction-specific protein ZO-1 was detected in the cultured NPE.

CONCLUSIONS

We developed a simple and reliable way to isolate, culture, and maintain NPE cells from porcine eyes. Success of the method hinged on our ability to isolate pure NPE in large number, detach the cells from the vitreous, and seed the cells at high density. The cultured cells express several proteins that are characteristic of native NPE. NPE cells cultured in this way may prove to be valuable for the study of ciliary body function.

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.