Expression of angiogenesis factors in human umbilical vein endothelial cells and their regulation by PEDF

The VEGFs and FGF-2 stimulate angiogenesis. Pigment epithelium-derived factor (PEDF) and thrombospondin-1 (TSP-1) strongly inhibit angiogenesis. Human umbilical vein endothelial cells (HUVECs) expressed VEGF-A, -B, -C, the VEGF receptors R1, R2, and R3, PEDF, FGF-2, and TSP-1, but VEGF-D transcripts were barely detectable. Hypoxia reduced the transcript levels of VEGF-C and its cognate receptor, VEGF-R3. PEDF blocked the effect of CoCl(2) on these two factors. The expression of VEGF-A and -B as well as VEGF-R1 and VEGF-R2 remained unchanged after exposure to hypoxia, PEDF, or both. There was a marked reduction in TSP-1 transcripts in CoCl(2) treated cultures and PEDF blocked this reduction. PEDF induced a small increase in FGF-2 transcripts in HUVECs, but there was no change in FGF-2 expression in HUVECs exposed to hypoxia or hypoxia plus PEDF. PEDF may control neovascularization, in part, by restoring the negative effects of hypoxia on the expression of a potent angiogenesis inhibitor, TSP-1. PEDF may also modulate vascular leakage by maintaining the transcriptional levels of the vascular homeostasis factors, VEGF-C and VEGF-R3 in hypoxic conditions.

Regulation of factors controlling angiogenesis in liver development: a role for PEDF in the formation and maintenance of normal vasculature

PEDF and VEGF are important inhibitors and promoters of angiogenesis, and the ratio between the two is an important indicator in many neovascular diseases. In mouse liver PEDF and VEGF(165) were co-expressed at very early stages of liver development and their expression increased as liver embryogenesis progressed, suggesting that PEDF and VEGF are both crucial to vasculogenesis as well. VEGF(189) only appears at the P0 stage in liver organogenesis and is maintained at high levels thereafter. PEDF and the two VEGF isoforms are synthesized by fresh and cultured hepatocytes. Expression of VEGF(121) and overexpression of VEGF(165) were only seen in HepG2, a well-characterized hepatocellular carcinoma line. The results suggest that hepatic vascular architecture is under the control of both PEDF and VEGF, and that VEGF(165) and VEGF(189) have distinct functions in normal vascular development of the liver. The VEGF isoforms 121 and 189 may be key regulators of increased vascularity and progression of hepatocellular carcinoma, one of the most common malignant tumors, and may be of prognostic significance for this tumor.

Retinoic acid and dexamethasone regulate the expression of PEDF in retinal and endothelial cells

Both all-trans-retinoic acid (ATRA) and pigment epithelial-derived factor (PEDF) regulate cell proliferation and differentiation. Treatment of human Y-79 retinoblastoma and A-RPE 19 pigment epithelial cells with ATRA increased the levels of PEDF protein and RNA. Endothelial cells from bovine retina and human umbilical cord expressed PEDF and the levels were also increased by ATRA. Mouse Müller glial cells and rat C6 glioma cells showed at least a 2.5 fold increase in PEDF RNA levels after ATRA treatment, as measured by quantitative PCR. The PEDF promoter contains a retinoic acid receptor element (RARE). Plasmids containing a PEDF promoter regulating a luciferase gene were transfected into D407 and C6 cells and the luciferase activity measured after incubation in the presence or absence of ATRA. In both cell types ATRA increased the level of luciferase activity suggesting the RARE is functional. Dexamethasone was also effective at increasing PEDF RNA levels in both mouse Muller glial cells and C6 rat glioma cells. To test the effects of PEDF on retinoic acid function, expression of retinoic acid receptors in Y-79 and A-RPE 19 cells was measured by PCR. In Y79 cells, PEDF treatment increased the expression levels of RARalpha and RXRgamma receptors and in the A-RPE 19 cells it resulted in a decrease in expression of the RARbeta and RXRbeta receptors. This study clearly indicates an interaction between PEDF and ATRA. The cell differentiation activities of PEDF may operate through mechanisms orchestrated by retinoids, and the converse may also be true. The differentiation, anti-mitotic, and apoptotic actions of PEDF and ATRA may utilize parallel pathways that converge at key junctional transduction molecules to coordinate cellular quiescence and maintain tissue mass in the presence of signals that stimulate abnormal cell proliferation. It will be an interesting therapeutic strategy to co-administer PEDF and retinoic acid in developing protocols for neovascular diseases in the eye and in cancer.

Osteoblasts and osteoclasts express PEDF, VEGF-A isoforms, and VEGF receptors: possible mediators of angiogenesis and matrix remodeling in the bone

Pigment epithelial derived factor (PEDF) is one of the most effective inhibitors of angiogenesis described so far, especially in controlling the growth of blood vessels in the eye. We now describe the localization of PEDF in regions of active bone formation in the mid-gestation mouse embryo and its specific and high levels of secretion by osteoblasts. PEDF is detected to a lesser extent in osteoclasts as well. The proangiogenic factors, VEGF and its receptors VEGF-R1 and VEGF-R2, are also expressed by both osteoblasts and osteoclasts. These findings suggest that bone angiogenesis and matrix remodeling may be mediated both by PEDF and by VEGF.

Developmental expression of intracellular targets of cGMP in rat visual cortex and alteration with dark rearing

We describe the temporal pattern of mRNA expression of some of the molecular components of the NO/cGMP second messenger system in the developing rat visual cortex and the effect of dark rearing on their expression levels using semiquantitative RT-PCR. mRNA expression for these molecules was altered by dark rearing in one of three ways: (1) no change--rod, olfactory, and cone/testis CNG channels, nonselective cation channels gated by cyclic nucleotides and highly permeable to Ca2+; (2) decrease--cyclic nucleotide phosphodiesterases which regulate cyclic nucleotide levels, and soluble guanylyl cyclase, the key synthetic enzyme producing cGMP and potently activated by nitric oxide; and (3) increase--cGMP kinase 1, a key enzyme activated by cGMP to phosphorylate a variety of intracellular proteins including cytoskeletal elements. These data suggest important and distinct roles for the cGMP system in both early and late developmental events in the rat visual cortex.

Defects in the MITF(mi/mi) apical surface are associated with a failure of outer segment elongation

The loss of MITF function in the MITF(mi/mi)mouse affects not only RPE differentiation, but also the development of rod photoreceptor outer segments. Our data indicate that opsin immunoreactivity is detected in the cell membrane and along the ONL/RPE border of developing MITF(mi/mi)rod photoreceptors and that rod outer segment morphogenesis is initiated. Although molecules associated with the outer segment continued to be expressed, outer segments did not elongate and develop stacked organized discs perpendicular to the RPE. The MITF(mi/mi)RPE also failed to form apical microvilli and lacked the apical network of the phosphoprotein ezrin seen in wild type tissue. The MITF(mi/mi)RPE basal surface was loosely organized and retained ezrin labelling which indicated some degree of differentiation. The correlation seen in our data suggest that there may be a link between the failure of the RPE apical domain to form and lack of rod outer segment elongation.

Miniature postsynaptic currents depend on Ca2+ released from internal stores via PLC/IP3 pathway

Miniature postsynaptic currents (mPSCs) were examined on autaptic innervation of single rat retinal ganglion cells in low density cultures. Removal of Ca2+ from bath solution or blocking of Ca2+ channels by Cd2+ had no detectable effect on mPSC frequency or amplitude. Thapsigargin, an agent for mobilization of Ca2+ from internal stores, increased mPSC frequency 3-5-fold in control, Ca2+-free or Cd2+-containing solutions. The inositol 1,4,5-triphosphate (IP3) receptor antago- nist, heparin; the phospholipase C (PLC) inhibitor, U73122; and caffeine abolished mPSC or decreased mPSCs frequency. Calcium imaging showed that cytosolic Ca2+ was increased by thapsigargin and decreased by caffeine. These data demonstrate that internal store-released Ca2+ regulated by the PLC/IP3/IP3-receptor pathway has critical contribution to generation and control of miniature release in retinal ganglion cells.

Müller cell protection of rat retinal ganglion cells from glutamate and nitric oxide neurotoxicity

PURPOSE

Low concentrations of excitotoxic agents such as glutamate and nitric oxide decrease survival rates of purified retinal ganglion cells (RGCs). In the retina, RGCs are ensheathed by retinal Müller glial (RMG) cell processes. The purpose of this study was to determine whether RMG cells could protect RGCs from these excitotoxic injuries.

METHODS

RGCs were purified from 7- or 8-day-old Long Evans rats and cultured on polylysine/laminin-coated coverslips in serum-free medium for 2 days. The coverslips were then moved to dishes containing either confluent RMG monolayers or no glial cells in glutamate-free medium. Some dishes with confluent RMG cells were exposed to D,L-threo-beta-hydroxyaspartate (THA), a blocker of glutamate uptake. Three days after exposure to various concentrations of glutamate or the NO donor, 2, 2'-(hydroxynitroso-hydrazino)bisethanamine, survival rates of RGCs were measured by calcein-acetoxymethyl ester staining. Glutamate concentrations in the medium were measured using amino acid analysis.

RESULTS

Without RMG cells, the application of increasing concentrations (5-500 microM) of glutamate caused a dose-dependent increase in RGC death after 3 days. The neurotoxic effects of glutamate were blocked in the RMG cell cocultures, even when there was no direct contact between the cell types. The protective effect of RMG cells was weakened by THA treatment. NO also had toxic effects on RGC. RMG cells prevented this toxicity but only when in direct contact with the RGCs.

CONCLUSIONS

RMG cells can protect RGCs from glutamate and NO neurotoxicity. We suggest that functional disorders of glutamate uptake in RMGs might be one of the etiologies of glaucoma.

The subcellular localization of Otx2 is cell-type specific and developmentally regulated in the mouse retina

Recent evidence implicates homeodomain-containing proteins in the specification of cell fates in the central nervous system. Here we report that in the embryonic mouse eye Otx2, a paired homeodomain transcription factor, was found in retinal pigment epithelial cells and a restricted subset of retinal neurons, including ganglion cells. In the postnatal and adult eye, however, both the cellular and subcellular distribution of the Otx2 protein were cell type-specific. Otx2 was detected only in the nuclei of retinal pigment epithelial and bipolar cells, but was present in the cytoplasm of rod photoreceptors. Immunohistochemical studies of retinal explants and transfected cell lines both suggested that the retention of Otx2 in the cytoplasm of immature rods is a developmentally regulated process. The differential distribution of Otx2 in the cytoplasm of rods and the nucleus of other cell types, suggests that subcellular localization of this transcription factor may participate cell fate determination during specific phases of retinal development.

Impairment of rod cGMP-gated channel alpha-subunit expression leads to photoreceptor and bipolar cell degeneration

PURPOSE

To determine whether alterations in rod cGMP-gated channel function mediate retinal degeneration, a transgenic approach in which the alpha subunit of the rod cGMP-gated channel is reduced by the expression of an antisense RNA was used.

METHODS

A 890-bp fragment of the 5' mouse rod cGMP-gated channel cDNA was cloned in the antisense orientation under the control of the strong bacterial cytomegalovirus promoter. This antisense construct was used to generate transgenic mice in which the expression of the antisense transgene was measured by reverse transcription-polymerase chain reaction. Histologic, immunocytochemical, and TdT-dUTP terminal nick-end labeling (TUNEL) analyses were performed on control and transgenic retina sections to determine the effects of antisense expression on specific cell types.

RESULTS

The antisense RNA was able to inhibit the translation of the rod channel protein in an in vitro assay. Three transgenic mouse lines expressing the antisense construct were obtained. Molecular analyses showed that the antisense is expressed in the eye of each transgenic mouse line, where it reduces rod cGMP-gated channel RNA expression. Histologic and immunocytochemical data showed that transgenic retinas have a reduced number of photoreceptors and bipolar cells. TUNEL staining confirmed that photoreceptor and bipolar cells degenerate along an apoptotic pathway.

CONCLUSIONS

Impairment of rod cGMP-gated channel alpha subunit expression leads to photoreceptor and bipolar cell degeneration. These transgenic mice are the first model of retinal degeneration caused by an alteration in the expression of the rod cGMP-gated channel. This model system can be used to test therapies designed to slow or stalled retinal degenerations.

Dorsal retinal pigment epithelium differentiates as neural retina in the microphthalmia (mi/mi) mouse

PURPOSE

Microphthalmia, a bHLH-zip transcription factor associated with the onset and maintenance of pigmentation, identifies the retinal pigment epithelial (RPE) compartment during optic vesicle and optic cup development. To determine a role for microphthalmia (mi) during eye development, the effects of an mi loss of function mutation on RPE and neural retinal were investigated in the mi/mi mouse.

METHODS

A series of embryonic and postnatal mi/mi and wild-type eyes were sectioned and labeled with neural retina- and RPE cell type-specific antibodies. Photoreceptor loss was quantified by counting the number of photoreceptor nuclei spanning the outer nuclear layer throughout postnatal retinal development.

RESULTS

Early neural retinal differentiation is not affected in the mi/mi mouse. The mi/mi ventral retinal pigment epithelial layer begins to develop normally, but does not pigment or attain a differentiated cuboidal morphology. The dorsal region of mi/mi retinal pigment epithelium expands and forms an ectopic retina, which develops all major retinal cell types along a similar time course as the wild type. After birth, mi/mi photoreceptors begin to form rosettes, outer segments fail to elongate, and over an extended time period, the retina degenerates.

CONCLUSIONS

Together these results suggest that early retinal development can proceed normally in the mi/mi mutant, but later retinal histogenesis is dependent on the presence of a differentiated retinal pigment epithelium. Most importantly, loss of mi function permits a change in cell fate from RPE to retina in the dorsal eye.

Temporal and spatial pattern of expression of cyclic nucleotide-gated channels in developing rat visual cortex

Cyclic nucleotide-gated channels, ligand-gated and highly permeable to calcium, are good candidates for transducing signals received by migrating cells, growth cones and developing synapses. The level of calcium in growth cones is important for axon guidance. Further, cyclic nucleotides, whose levels can be altered by nitric oxide and other transmitters, are known to alter growth cone motility. We use rat visual cortex as a model in our semi-quantitative RT-PCR and in situ hybridization studies to determine the developmental time course and localization of all three CNG family members (rod, olfactory and cone/testis). We demonstrate that in the cortex, the three channel subtypes are each expressed in a distinct temporal and spatial pattern in only sensorimotor and occipital regions of the cortex. Specifically, the rod and olfactory subtypes are present at the time of migration and rapid dendritic outgrowth, and the cone/testis subtype is highly expressed after eye opening. These results suggest CNG channels may play a role in both early and late events in visual cortical development.

Widespread expression of olfactory cyclic nucleotide-gated channel genes in rat brain: implications for neuronal signalling

The cyclic nucleotides cAMP and cGMP are important intracellular messengers involved in a wide variety of signal transduction events in the nervous system. It has been proposed that cAMP/cGMP elicit some of their effects through direct gating of a novel class of Ca2+ -permeable ion channels that are termed cyclic nucleotide-gated (CNG) channels. Previous studies have identified the expression of a gene encoding one major CNG channel subtype, the olfactory receptor neuron alpha subunit, in the brain [El-Husseini et al. (1995) NeuroReport 6:1331-1335; Kingston et al. (1996a) Proc. Natl. Acad. Sci. U.S.A. 93:10440-10445; Bradley et al. (1997) J. Neurosci. 17:1993-2085]. We, therefore, proposed that the actions of cAMP/cGMP on neurons in the brain might occur through the activation of these CNG channels. To determine how widespread such a function might be, the regional and cellular distribution of the olfactory CNG channel alpha subunit has been examined in detail. Primers for multiple portions of the olfactory CNG channel were used in polymerase chain reaction (PCR) to amplify cDNA reverse-transcribed from several brain regions. The identities of PCR products were confirmed with Southern blots and by sequencing. In situ hybridization experiments demonstrated localization of CNG channel mRNA in discrete neuronal populations throughout the brain. In agreement with previous work, relatively strong hybridization signals are present in neuronal cell bodies of the cerebellum, olfactory bulb, cerebral cortex, and brainstem. Additionally, somewhat lesser signals are found in thalamus, hypothalamus, midbrain, and spinal cord while no hybridization signal was detectable in the caudate nucleus. This surprisingly wide distribution throughout the rat brain strengthens the hypothesis that CNG channels may influence numerous processes as downstream effectors of cyclic nucleotide cascades. Interestingly, the distribution of CNG channels is very similar to that of the nitric oxide/cGMP system, suggesting that one function of CNG channels in the brain could be to link diffusible messengers to elevated Ca2+ entry into neurons.

Molecular and pharmacological analysis of cyclic nucleotide-gated channel function in the central nervous system

Most functional studies of cyclic nucleotide-gated (CNG) channels have been confined to photoreceptors and olfactory epithelium, in which CNG channels are abundant and easy to study. The widespread distribution of CNG channels in tissues throughout the body has only recently been recognized and the functions of this channel family in many of these tissues remain largely unknown. The molecular biological and pharmacological properties of the CNG channel family are summarized in order to put in context studies aimed at probing CNG channel functions in these tissues using pharmacological and genetic methods. Compounds have now been identified that are useful in distinguishing CNG channel activated pathways from cAMP/cGMP dependent-protein kinases or other pathways. The ways in which these interact with CNG channels are understood and this knowledge is leading to the identification of more potent and more specific CNG channel subtype-specific agonists or antagonists. Recent molecular and genetic analyses have identified novel roles of CNG channels in neuronal development and plasticity in both invertebrates and vertebrates. Targeting CNG channels via specific drugs and genetic manipulation (such as knockout mice) will permit better understanding of the role of CNG channels in both basic and higher orders of brain function.

Erx, a novel retina-specific homeodomain transcription factor, can interact with Ret 1/PCEI sites

Our previous studies on the transcriptional regulation of rod opsin gene expression had defined a strikingly conserved element, Ret 1/PCEI, present in the upstream regulatory regions of opsin and other photoreceptor-specific genes. This element interacts with a 40 kDa, developmentally regulated, retina-specific protein. In this study we report the cloning of the novel retina-specific homeodomain protein Erx. Erx contains a homeodomain that is 79% homologous to that of Drosophila empty spiracles. This 40 kDa protein can interact with the Ret 1 element in electrophoretic mobility shift assays. Mutation of key residues in Ret 1 eliminates all Erx binding. Transient transfection of Y79 retinobalstoma cells with Erx leads to significant transcriptional activation of a reporter gene via Ret 1 elements. We conclude that Erx is the Ret 1 binding activity. This is the first example of a Q50 homeodomain protein expressed in retinal photoreceptors.

Developmental expression of the rat rod photoreceptor cGMP-gated cation channel

The appearance of cGMP-gated cation channel protein in the postnatal rat retina has been studied by fluorescence immunocytochemistry of radial retinal sections and immunoblots of retinal membrane proteins. Channel immunoreactivity was first detectable with RCNGC1-7H2 monoclonal antibody at postnatal day 7 (PN7) by both methods. Immunocytochemical label in retinal sections was localized to the outer segments, and immunoreactivity increased with increasing age. We also compared the developmental appearance of the cGMP-gated cation channel to that of other phototransduction proteins and developmental markers. RET-P2, a monoclonal antibody recognizing the 39-kDa rds/peripherin disc protein, first labeled outer segments at PN7, coincident with cGMP-gated cation channel expression. Double labeling of the same section of PN7 rat retina with RET-P2 and R309 (a polyclonal antiserum against the rod cGMP-gated cation channel) revealed identical patterns of labelling. Similarly, double labeling with RCNGC1-7H2 and an antibody against the rod cGMP-phosphodiesterase gave coincident labeling, suggesting coordinate expression mechanisms of phototransduction proteins with each other and with outer segment structural proteins.

HPC-7: a novel oligodendrocyte lineage protein which appears prior to galactocerebroside

The identification of cell type-specific molecules expressed at different developmental stages can help to elucidate the regulatory mechanisms governing the survival, differentiation, and development of cells in the central nervous system (CNS). A cell surface protein, HPC-7, was detected on rat oligodendrocytes (OL) in culture by a monoclonal antibody generated against adult rat hippocampal membranes. Adult rat brain and sciatic nerve sections showed selective labeling of white matter and other myelinated fibers in both the CNS and peripheral nervous system (PNS). Double-labeling of secondary cultures of OL, O-2A, and type-2 astrocytes and primary cultures of type-1 astrocytes with independent cell type-specific antibodies confirmed that HPC-7 was expressed only by the OL lineage. By using a series of OL stage-specific antigenic markers (A2B5, 04, OL-1, galactocerebroside, myelin basic protein) HPC-7 was found to appear at the time when OL precursors became A2B5 negative and began their terminal differentiation in OL. On immunoblots, anti-HPC-7 antibody recognized a single 66 kDa band in rat OL and a single band at 100 kDa in adult myelin. N-glycosidase treatment showed that the HPC-7 protein did not contain substantial amounts of N-linked carbohydrate. Thus, HPC-7 appears to be a cell surface protein of the OL lineage that marks the important transition from proliferative precursor to postmitotic OL.

Neurotoxic effects of low doses of glutamate on purified rat retinal ganglion cells

PURPOSE

To determine whether low concentrations of glutamate induce cell death in purified rat retinal ganglion cells (RGCs).

METHODS

Rat retinal ganglion cells were purified from dissociated retinal cells by a modified two-step panning method and were cultured in serum-free medium containing neurotrophic factors and forskolin. Survival of RGCs after exposure to glutamate, with or without glutamate receptor antagonists, was measured by calcein-acetoxymethyl ester staining after 3 days in culture. To visualize calcium signals, RGCs were loaded with the calcium indicator dye, fluo-3 acetoxymethyl ester, and fluorescence was measured by laser scanning confocal microscope. Electrophysiological properties of RGCs were examined by using the whole-cell, patch-clamp technique.

RESULTS

The application of increasing concentrations (5-500 microM) of glutamate caused a dose-dependent increase in RGC death after 3 days in culture. Neurotoxic effects of low doses of glutamate were totally blocked by a specific alpha-amino-3-dihydro-5-methyl-isoxazol-4-propionic acid-kainate (AMPA-KA) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), but not by a specific N-methyl-D-aspartate receptor antagonist, 2-amino-5-phosphonovalerate (APV). In addition, calcium imaging and patch-clamp recordings showed that intracellular calcium accumulation and glutamate-evoked inward currents were completely blocked by DNQX but not by APV.

CONCLUSIONS

Low doses of glutamate can activate AMPA-KA receptors in RGCs, which causes increases in intracellular calcium and decreases in cell survival. This is the first report to show the functional role of calcium-permeable AMPA-KA receptors in cultured RGCs.

Substituted cGMP analogs can act as selective agonists of the rod photoreceptor cGMP-gated cation channel

Cyclic nucleotide-gated (CNG) channels are expressed in many cell types in both the nervous system and nonexcitable tissues. In order to understand the roles of cGMP-gated channels, and to distinguish actions of cGMP mediated through CNG channels from those through cGMP-dependent protein kinase (G-kinase), several new cGMP analogs were tested for potency as CNG channel agonists. Using Xenopus oocytes expressing the rat rod cGMP-gated ion channel alpha-subunit, we showed that an analog containing a pCPT group at the 8-position, 8-pCPT-cGMP, was 80 times more potent than cGMP and 14 times more potent than 8-Br-cGMP. 8-pCPT-cGMP is the most potent CNG channel agonist so far described and also has the advantages of much better membrane permeability as well as much higher resistance to PDE-hydrolysis, as compared with 8-Br-cGMP. Modification of both 8-Br-cGMP and 8-pCPT-cGMP by introduction of a sulphur atom into the cyclic phosphate group gave smaller changes in agonist efficiency. Both Sp-8-Br-cGMPS and Sp-8-pCPT-cGMPS acted as agonists of CNG channels and are also G-kinase activators. In contrast, Rp-8-Br-cGMPS was a channel agonist, with an EC50 of 173.5 microM, but a G-kinase antagonist with a Ki of 4 microM. Finally, Rp-8-pCPT-cGMPS was a channel agonist and showed additional noncompetitive antagonist activity at higher concentrations. The results suggest that 8-pCPT-cGMPS is a highly potent photoreceptor CNG channel agonist with high membrane permeability and PDE-resistance and furthermore Rp-8-Br-cGMPS can be used to test whether the actions of cGMP are selectively mediated by CNG channels.

Direct blockade of both cloned rat rod photoreceptor cyclic nucleotide-gated non-selective cation (CNG) channel alpha-subunit and native CNG channels from Xenopus rod outer segments by H-8, a non-specific cyclic nucleotide-dependent protein kinase inhibitor

Using excised inside-out patch techniques, a non-specific cyclic nucleotide-dependent protein kinase inhibitor, H-8 (N-2-(methylamino)ethyl-5-isoquinolinesulfonamide), has been shown to suppress directly the activities of both rat rod photoreceptor cyclic GMP-gated channel alpha-subunits expressed in Xenopus oocytes and native cGMP-gated channels from Xenopus rod outer segments. When co-applied with 100 microM cGMP on the cytoplasmic side of the patches, current suppression by H-8 increased with membrane depolarization. Blockade by H-8 was not relieved by supersaturating concentrations of cGMP (1 mM). In addition, blockade by H-8 showed significant inhibition of channel activity at negative holding potentials when acting from the extracellular side of the channel. The results were consistent with the conclusion that H-8 can act as an open channel blocker from either side of the channel. Even though H-8 is a much more effective kinase inhibitor, it is frequently used at concentrations that cause inhibition of cGMP-gated channels and this effect needs to be taken into account when evaluating results obtained with this compound.

Cyclic nucleotide gated channels as regulators of CNS development and plasticity

Cyclic nucleotide gated (CNG) cation channels are critical for signal transduction in vertebrate visual and olfactory systems. Members of the CNG channel gene family have now been cloned from a number of species, from Caenorhabditis elegans to humans. An important advance has been the discovery that CNG channels are present in many neurons of the mammalian brain. CNG channels act as molecular links between G-protein-coupled cascades, Ca2+-signalling systems, and gaseous messenger pathways. Perhaps most striking are recent data implicating CNG channels in both developmental and synaptic plasticity.

Differentiation and transdifferentiation of the retinal pigment epithelium

The retinal pigment epithelium (RPE) lies between the retina and the choroid of the eye and plays a vital role in ocular metabolism. The RPE develops from the same sheet of neuroepithelium as the retina and the two derivatives become distinguished by different expression patterns of a number of transcription factors during embryonic development. As the RPE layer differentiates it expresses a set of unique molecules, many of which are restricted to certain regions of the cell. PRE cells undergo both a loss of polarity and a loss of expression of many of these cell type-specific molecules when placed in monolayer culture. The RPE of many species, including mammals, can be induced to transdifferentiate by growth factors such as basic fibroblast growth factor. Under the influence of such factors the RPE is triggered to alter expression of a wide array of molecules and to take on a retinal epithelium fate, from which differentiated retinal cell types including rod photoreceptors can be produced.

Experimental preretinal neovascularization by laser-induced venous thrombosis in rats

PURPOSE

Retinal ischemia and neovascularization (NV) are important components of many retinal disorders. To facilitate further investigation of retinal ischemia and neovascularization, we sought to develop a reproducible in vivo experimental model of venous occlusion by photodynamic thrombosis in rats.

METHODS

After anesthesia, 27 eyes of pigmented rats received an intraperitoneal injection of 0.2 ml of, 10% sodium fluorescein 15 minutes prior to laser treatment. With a blue-green argon laser, selected venous sites next to the optic nerve head were photocoagulated indirectly with a 78 diopter lens. Venous occlusion was accomplished using laser parameters of 1.0 second, 50 microns, and 50-100 mW. For a control group, 10 eyes were coagulated on the retina between major vessels using the same parameters after fluorescein injection. For a second control group, 1% sodium hyaluronate was injected into the subretinal space to make a long-standing retinal detachment in 5 eyes.

RESULTS

With 1-8 laser impulses, each venous occlusion was obtained and was associated with extreme venous constriction and tortuousity. Retinal edema became evident 10-30 minutes after treatment in the sectors associated with the occluded veins. This edema became a bullous retinal detachment (RD) within 12 hours and intra-retinal hemorrhage was observed. The retinal edema continued for 3-10 days and the retinas reattached spontaneously. Prior to or after retinal reattachment 70% (19/27) of eyes developed retinal NV and tractional RD. Of these, 11 developed NV of the optic disc (NVD), 6 developed NV elsewhere (NVE), and 2 developed NVD and NVE. In 30% (8/27) of the eyes, retinal edema resolved without evidence of NV. In control groups no eyes showed either circulatory disorders or evidence of NV.

CONCLUSIONS

This is a new model of retinal ischemia and associated neovascularization established by venous thrombosis that is easily reproducible. Many aspects of rat retinal physiology are known and this model has promise as an avenue for further investigation.

Identification of competitive antagonists of the rod photoreceptor cGMP-gated cation channel: beta-phenyl-1,N2-etheno-substituted cGMP analogues as probes of the cGMP-binding site

cGMP is the natural activator of the cyclic nucleotide-gated channel originally isolated from rod photoreceptors but now known to be expressed in a wide variety of neural and non-neural cells. To identify antagonists of cGMP action and to better understand the interaction between cGMP and the channel protein, experimental studies were undertaken using four synthetic cGMP analogues, PET-cGMP, 8-Br-PET-cGMP, Rp-8-Br-PET-cGMPS, and Sp-8-Br-PET-cGMPS. With excised patches from either Xenopus oocytes expressing a cloned rat rod channel alpha-subunit or from native Xenopus rod photoreceptors, Rp-8-Br-PET-cGMPS competitively suppressed the cGMP-induced current with an IC50 of 25 microM and Sp-8-Br-PET-cGMPS inhibited this current with an IC50 of 105 microM. On the expressed rat rod channel, 8-Br-PET-cGMP behaved as a very weak partial agonist at high concentrations and an antagonist (IC50 = 64 microM) at lower concentrations when coapplied with cGMP. PET-cGMP did not activate channel currents alone but showed a synergism when coapplied with subsaturating concentrations of cGMP. Because Sp-8-Br-PET-cGMPS is a potent activator of type I cGMP-dependent protein kinase, but a competitive antagonist of channel activation, it will be a useful reagent for discriminating between those effects of cGMP that are mediated by a protein kinase and those mediated by channel activation. Because the PET derivatives all contain a phenyl-substituted 5-membered ring system fused to the amino group in position 2 and the nitrogen in position 1 of the guanine ring, the results support the idea that N1 and N2 are important for channel activation. They also suggest a minor role for the cyclic phosphate group in binding or activation.

cGMP-mediated effects on the physiology of bovine and human retinal Müller (glial) cells

1. Whole-cell currents of freshly dissociated or cultured Müller cells from human and bovine retinas were studied using the perforated-patch and standard whole-cell recording techniques. 2. We found that internal perfusion of cGMP or external exposure to 8-bromo-cGMP activated a calcium permeable, non-selective cation current in Müller cells, the principal glial cells of the retina. In addition, the activity of calcium-activated potassium channels increased markedly. These currents were minimally affected by cAMP. 3. Molecular studies using the reverse transcription-polymerase chain reaction demonstrated that human müller cells in culture contain transcripts closely related to the rod cyclic nucleotide-gated (CNG) channel. 4. Since guanylate cyclase is a known target for nitric oxide (NO), we tested the effect of NO donors on Müller cell currents. These agents induced currents that were qualitatively similar to those activated by cGMP. 5. Our experiments support the idea that the NO-cGMP pathway regulates the physiology of Müller cells and may play a role in integrating neuron-glia interactions in the retina.

Ret 1, a cis-acting element of the rat opsin promoter, can direct gene expression in rod photoreceptors

The Ret 1 element, located at -136 to -110 in the rat opsin promoter, binds developmentally regulated retinal nuclear proteins. A similar sequence is found up-stream of opsin genes, from humans to Drosophila, as well as many other photoreceptor-specific genes. The function of the Ret 1 element was tested both in vitro and in two sets of transgenic mice. A mutated Ret 1 element did not bind retinal nuclear proteins in vitro. The same mutations in an otherwise normal 1.9-kb rat opsin promoter failed to drive expression of a lacZ reporter gene in nine of 12 lines. In the three other lines, expression in photoreceptors was very faint. Four tandem copies of the Ret 1 element maintained the Ret 1 binding specificity in vitro and were able to direct expression of a lacZ transgene in photoreceptors of all nine mouse lines obtained. In two lines, expression was also detected in the ganglion cell layer and the ciliary epithelium. In three lines, a characteristic pattern of expression was found in the nervous system in addition to the normal retinal expression. These results indicate that Ret 1 can and is necessary to drive gene expression in rod photoreceptors. Furthermore, our results suggest that Ret 1-like elements may also be important in the developing nervous system.

Rat hippocampal neurons express genes for both rod retinal and olfactory cyclic nucleotide-gated channels: novel targets for cAMP/cGMP function

Cyclic nucleotide-gated (CNG) channels are Ca(2+)-permeable, nonspecific cation channels that can be activated through direct interaction with cAMP and/or cGMP. Recent electrophysiological evidence for these channels in cultured hippocampal neurons prompted us to investigate the expression of CNG channel genes in hippocampus. PCR amplification detected the expression of transcripts for subunit 1 of both the rod photoreceptor (RCNGC1) and the olfactory receptor cell (OCNGC1) subtype of CNG channel in adult rat hippocampus. In situ hybridization detected expression of both channel subtypes in most principal neurons, including pyramidal cells of the CA1 through CA3 regions and granule cells of the dentate gyrus. From the hybridization patterns, we conclude that the two genes are colocalized in individual neurons. Comparison of the patterns of expression of type 1 cGMP-dependent protein kinase and the CNG channels suggests that hippocampal neurons can respond to changes in cGMP levels with both rapid changes in CNG channel activity and slower changes induced by phosphorylation. Future models of hippocampal function should include CNG channels and their effects on both electrical responses and intracellular Ca2+ levels.

Differential effects of bFGF on development of the rat retina

A variety of growth factors can influence the expression of differentiated properties by cell types of the developing retina. One unresolved question has been whether these factors can direct the differentiation pathway of uncommitted precursors or whether they act to help the expression of properties by already committed cells. To address this question we have studied the effects of basic fibroblast growth factor (bFGF) on the differentiation of ganglion cells and rod photoreceptors in explant cultures of embryonic rat retinas. Incubation of retinas in the presence of bFGF accelerated the appearance of differentiated ganglion cells and incubation in the presence of anti-bFGF antibodies delayed the appearance. bFGF had no effect on the appearance of differentiated rod photoreceptors as judged by expression of opsin, although all-trans-retinoic acid did increase the number of cells expressing opsin. bFGF inhibited the formation of rod photoreceptor rosettes suggesting that it does influence some properties of rods or the adjacent Müller glial cells. The results suggest that bFGF can alter the timing of differentiation of retinal ganglion cells but not direct their production from retinal precursors.

Retinal Thy-1 expression during development

PURPOSE

To evaluate the developmental expression of Thy-1 in the retina. Thy-1, the most abundant mammalian neuronal surface glycoprotein, is likely to play a significant role in retinal development. In the mammalian retina, it is found predominantly, if not exclusively, on retinal ganglion cells.

METHODS

Rat retinae of various ages were stained immunohistochemically for Thy-1 with 2G12, a monoclonal Thy-1 antibody. Sections were analyzed digitally to quantify bound antibody. Using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR), the expression of Thy-1 protein was compared with the levels of mRNA detected.

RESULTS

Thy-1-dependent fluorescence was detected in rat retinae from birth, albeit at low levels. Thy-1 labeling was localized predominantly to the ganglion cell layer. Minimal, fine patterns of linear and reticular fluorescence were noted in the inner nuclear layer. Thy-1 levels reached a maximal level at approximately postnatal day 14. RT-PCR measurements showed a similar time course for the increase in Thy-1 expression.

CONCLUSIONS

The Thy-1 antigen is present in the inner retina at birth. Its level increases steadily after birth and peaks during the second week of life. Thy-1 expression is approximately coterminous with synaptogenesis of the inner plexiform layer and may play a role in synaptogenesis of the inner retina or in other developmental milestones in the formation of the visual system.

Isolation and characterization of the alpha-subunit of the rat rod photoreceptor cGMP-gated cation channel

A combination of genomic and PCR clones has been used to derive the full-length coding sequence of the alpha-subunit of the rat rod photoreceptor cGMP-gated cation channel. The sequence encodes a protein of 683 amino acids with a predicted molecular weight of 79,221. The sequence shows extensive homology with other rod cGMP-gated channels and also with the rat olfactory cyclic nucleotide-gated cation channel. When the full-length sequence of the rat rod channel was expressed in Xenopus oocytes it gave a conductance that responded to cGMP with an EC(50) of 42 mu M. No response to 2 mM cAMP was detected. The conductance was decreased in the presence of increasing concentrations of calcium. Both monoclonal and polyclonal antibodies were generated against a C-terminal peptide of the rat rod channel. On immunoblots of adult rat retinal membranes the antibodies recognized a band of 71 kDa, suggesting that the rat channel may undergo proteolytic cleavage in the retina, as has previously been found for the bovine rod channel. Immunocytochemical labeling of adult rat retinal sections detected prominent labeling over the rod photoreceptor outer segments with both monoclonal and polyclonal antibodies.

An astrocytic binding site for neuronal Thy-1 and its effect on neurite outgrowth

Thy-1, a member of the immunoglobulin superfamily, is one of the most abundant glycoproteins on mammalian neurons. Nevertheless, its role in the peripheral or central nervous system is poorly understood. Certain monoclonal antibodies to Thy-1 promote neurite outgrowth by rodent central nervous system neurons in vitro, suggesting that Thy-1 functions, in part, by modulating neurite outgrowth. We describe a binding site for Thy-1 on astrocytes. This Thy-1-binding protein has been characterized by immunofluroesence with specific anti-idiotype monoclonal antibodies and by three competitive binding assays using (i) anti-idiotype antibodies, (ii) purified Thy-1, and (iii) Thy-1-transfected cells. The Thy-1-binding protein may participate in axonal or dendritic development in the nervous system.

A calcium-permeable cGMP-activated cation conductance in hippocampal neurons

Whole-cell patch clamp recordings detected a previously unidentified cGMP-activated membrane conductance in cultured rat hippocampal neurons. This conductance is nonselectively permeable for cations and is completely but reversibly blocked by external Cd2+. The Ca2+ permeability of the hippocampal cGMP-activated conductance was examined in detail, indicating that the underlying ion channels display a high relative permeability for Ca2+. The results indicate that hippocampal neurons contain a cGMP-activated membrane conductance that has some properties similar to the cyclic nucleotide-gated channels previously shown in sensory receptor cells and retinal neurons. In hippocampal neurons this conductance similarly could mediate membrane depolarization and Ca2+ fluxes in response to intracellular cGMP elevation.

Rat retinal Müller cells express Thy-1 following neuronal cell death

In the normal rat retina the Thy-1 antigen is a specific marker of ganglion cells, but degeneration of ganglion cells in vivo does not remove completely the expression of Thy-1 in the retina. To reconcile these differences we have postulated that ganglion cell death could induce a glial response including the expression of Thy-1 in Müller cells, the main glial cell type in the retina. Using immunocytochemistry, we have shown that pure cultures of Müller cells were strongly labelled with antibodies against Thy-1. PCR amplification of cDNA reverse transcribed from Müller cell RNA indicated the presence of Thy-1 transcripts. Double labelling experiments with anti-Thy1 and anti-glutamine synthetase, a marker of Müller cells, indicated the presence of both antigens in the same cells. Although Müller cells expressed Thy-1 mRNA and protein when cultured in the absence of neuronal cells, when co-cultured with retinal neurons they were not labelled with antibodies against Thy-1. Our results suggest that Thy-1 is expressed by Müller cells following loss of retinal neurons. Thy-1 may have an important function during glial response to neuron death in retina.

In vitro transdifferentiation of embryonic rat retinal pigment epithelium to neural retina

Divergence of neural retinal and retinal pigment epithelial (RPE) lineages from the optic vesicle neuroepithelium starts at a very early stage of eye development. Partially or even fully differentiated RPEs of some vertebrate species are capable of transforming into neural retina. In the present study, we have shown that mammalian RPE possesses the ability to transdifferentiate into neural retina at early embryonic stages. If cultured in serum-free medium, presumptive rat RPE became pigmented and expressed a molecular marker of mature RPE. In the presence of basic fibroblast growth factor (bFGF), cultured early embryonic rat RPE did not acquire pigment and grew to form retina-like multilayer structure containing neuronal cells and cells that express markers of retinal ganglion, amacrine and rod photoreceptor cells. The effects of bFGF occurred independently of effects on cell division and became irreversible after periods that varied with tissue age. This study has demonstrated that already differentiated embryonic rat RPE still retain the ability to become neural retina up to certain stage.

Treatment of retinal breaks with autologous serum in an experimental model

PURPOSE

The standard treatment for retinal breaks is thermal adhesion. Breaks in the posterior pole (i.e., macular holes) recently have been treated using vitrectomy and the recombinant cytokine transforming growth factor-beta. This has been shown to achieve closure of the retinal breaks by stimulating localized fibrocellular proliferation. Serum has been shown to contain chemoattractants and mitogens for many types of cells. The authors studied the clinical and histologic effect of autologous serum application to retinal breaks in an experimental model.

METHOD

Twenty-four rabbits underwent pars plana lensectomy, vitrectomy, retinectomy, fluid-air exchange, application of test solution (12 with Hank's buffered salt solution and 12 with autologous serum), and air-gas exchange. Clinical examination with indirect ophthalmoscopy was performed, and animals were killed 5, 14, and 28 days after treatment. Tissue sections through the retinectomy were studied by light microscopy, electron microscopy, and immunocytochemistry.

RESULTS

None of the serum-treated eyes showed retinal detachment at the site of the retinectomy by evaluation with indirect ophthalmoscopy at each of the time points. In contrast, in control eyes retinal detachment developed at the retinectomy site from 0% at day 5 to 50% at day 14 and 75% at day 28. By light microscopy, serum-treated eyes contained multilayers of fibroblast-like cells adhering the retinectomy edges to the underlying retinal pigment epithelium and choroid. The control eyes had nonadherent retinal edges at the retinectomy site with little sign of fibrocellular response. Results were confirmed by electron microscopy. The fibroblast-like cells by immunocytochemistry contained vimentin, cytokeratin 18, and/or glial fibrillary acidic protein.

CONCLUSION

This study suggests that serum induces a localized fibrocellular response at the retinectomy edges compared with control eyes. This response, characterized by light microscopy, electron microscopy, and immunocytochemistry, appears to involve a mixed population of glial, retinal pigment epithelial, and/or fibroblastic cells. These cells seem to enhance adhesion and subsequent reattachment of the edges of the retinectomies at the time points studied when compared with controls.

Retinal ganglion cell survival is promoted by genetically modified astrocytes designed to secrete brain-derived neurotrophic factor (BDNF)

Genetically engineered cells carrying genes for neurotrophic factors have potential application for treatment of neurodegenerative diseases and injuries to the nervous system. Brain-derived neurotrophic factor (BDNF) promotes the survival of specific neurons, including retinal ganglion cells (RGC). To determine whether genetically engineered astrocytes might be used for delivering bioactive BDNF, we infected primary type 1 rat astrocytes with a retrovirus harboring a human prepro-BDNF cDNA and assayed the medium conditioned by these astrocytes for effects on survival of rat RGCs in vitro. High levels of BDNF mRNA were expressed by infected astrocytes, but not by control astrocytes as determined by RNase protection assay using a BDNF specific probe. To test for secretion of bioactive BDNF from the transgenic astrocytes, embryonic day 17 rat retinas were dissociated and grown in medium conditioned (CM) for 24 h by astrocytes infected with a replication deficient retrovirus carrying BDNF, NGF, or alkaline phosphatase (AP) cDNA. After 3 days, the number of Thy-1 immunoreactive RGCs was counted. BDNF astrocyte CM significantly enhanced RGC survival by 15-fold compared to the AP control. NGF astrocyte CM had no significant effect. The rate of BDNF secretion was estimated at 83-166 pg/10(5) cells/h. This study demonstrates that astrocytes can be genetically engineered to synthesize and secrete bioactive BDNF. These techniques may be applicable to rescuing neurons from degenerative processes and also for enhancing their survival following transplantation.

Characterization and regulation of the protein binding to a cis-acting element, RET 1, in the rat opsin promoter

RET 1 is a binding site for retinal nuclear proteins located at -136 to -110 bp in the rat opsin promoter, as defined by DNase protection assays. A similar sequence is found in the upstream flanking regions of many other photoreceptor genes in mammals and other species, including Drosophila. A 7-base consensus sequence, CAATTAG, is found in these genes and has the binding activity of the longer RET 1 element. A 40-kDa protein that binds to RET 1 has been purified over 2 x 10(5)-fold to apparent homogeneity by affinity chromatography. The RET 1 binding activity is first detectable at E18 and increases during the first two postnatal weels, At embryonic ages the retarded bands show an altered mobility and at early postnatal ages two bands are detected, with the adult band increasing and the embryonic band decreasing in intensity. Treatment of early postnatal retinas with bFGF increased the binding activity in nuclear extracts and caused a shift in migration of the retarded band to a position characteristic of the embryonic form of the complex. The results support the hypothesis that RET 1-like elements play an important role in rod photoreceptor development.

A cis-acting element, T alpha-1, in the upstream region of rod alpha-transducin gene that binds a developmentally regulated retina-specific nuclear factor

The G protein transducin (T) is an integral component of the signal transduction pathway in photoreceptors. We have identified a cis-acting element, T alpha-1, in the upstream region of the mouse rod alpha-T (Tr alpha) gene that may be important for tissue-specific expression. T alpha-1 binds a retina-specific nuclear factor of apparent molecular mass of 90 kDa. Binding to the T alpha-1 site is developmentally regulated and peaks between postnatal days 6 and 9. This corresponds to the time of rod photoreceptor maturation and the rise in Tr alpha gene expression. The sequence of T alpha-1 shows homology with RET-1, a cis-acting element in the proximal promoter of opsin gene that binds a distinct retina-specific factor. T alpha-1 and RET-1 sequences may have been derived from a prototype T alpha-1/RET-1 sequence, evolved to confer photoreceptor specificity on retina-specific genes.

Retinal ganglion cells express a cGMP-gated cation conductance activatable by nitric oxide donors

We have identified a putative cGMP-gated cation conductance in rat retinal ganglion cells. Both in situ hybridization and polymerase chain reaction amplification detected transcripts in ganglion cells that were highly homologous to the cGMP-gated cation channel expressed in rod photoreceptors. Whole-cell patch-clamp recordings detected a current stimulated by cGMP due to activation of nonselective cation channels. This current had a reversal potential near 0 mV, showed some outward rectification, and could be blocked by Cd2+. The current could also be activated by a phosphodiesterase inhibitor and the nitric oxide donors sodium nitroprusside and S-nitrosocysteine. We propose that nitric oxide released from an identified subpopulation of amacrine cells may activate this channel to modulate ganglion cell activity.

Synaptic and photoreceptor components in retinal pigment epithelial cell transplanted retinas of Royal College of Surgeons dystrophic rats

Plexiform layer synaptic and photoreceptor cell components were investigated in retinas of Royal College of Surgeons (RCS) dystrophic rats transplanted with normal retinal pigment epithelial (RPE) cells by immunocytochemistry using previously characterized monoclonal antibodies. In retinas of normal adult rats and RPE-cell transplanted retinas of 4 month-old RCS rats, HNK-1, a marker for a carbohydrate of the neural cell adhesion molecule (N-CAM), was detected immunocytochemically in the inner and outer plexiform layers and ganglion cell bodies and their axons. HNK-1 was also detected in the inner plexiform layer of nontreated retinas of 4 month-old RCS rats, but was reduced to scattered patches in the outer plexiform layer. In addition, immunoreactivity for the SVP-38 antibody recognizing synaptophysin was found in both plexiform layers of normal adult rat retinas and RPE-transplanted retinas of 4 month-old RCS rats. Furthermore, photoreceptor cell bodies and their inner and outer segments were immunostained for the opsin monoclonal antibody RET-P1 in retinas of normal adult rats and RPE-cell transplanted retinas of 4 month-old RCS rats. However, in nontreated retinas of 4-month-old RCS rats, only immunostained debris material was detected. These results strongly suggest that normal RPE transplants not only rescue photoreceptor cells in RCS rats, but also maintain an essential functional capacity, in this case, synaptic components in the plexiform layers.

Glutamate and GABA in retinal circuitry

Glutamate and GABA have been identified as the major neurotransmitters in the radial and lateral synaptic pathways, respectively, of the vertebrate retina. Over the past year or so new information has appeared that has significantly increased the knowledge of how these compounds can elicit a range of responses. Key features of this new information are the identification and localization of many receptor subtypes within the retina, the recognition that glutamate can modulate membrane potential through cGMP-gated ion channels, and the finding that GABA can be released through non-vesicular pathways.

Shared nuclear protein binding sites in the upstream region of the rat opsin gene

DNase I protection and gel retardation assays have identified two sequences 5' to the rat opsin gene that interact with nuclear proteins from retina but not from a number of neuronal and non-neuronal tissues. These sites, Ret 2 and Ret 3, are over 1200 base pairs apart but seem to interact with the same protein(s). Synthetic oligonucleotides corresponding to each site were able to inhibit complex formation in a gel retardation assay using an oligonucleotide corresponding to the other site. The proteins binding to the Ret 2 and Ret 3 sites co-eluted in both ion exchange and gel filtration chromatography. The protein(s) were also present at adult levels at birth, suggesting that they may represent differentiation products expressed in the proliferating retinal epithelium.

RET-PE10: a 61 kD polypeptide epitope expressed late during vertebrate RPE maturation

PURPOSE

This study sought To learn more about the mechanisms that determine and maintain the differentiated state of the retinal pigment epithelium (RPE).

METHODS

Monoclonal antibodies were raised against human RPE and used in conjunction with other antibodies. Immunocytochemical and biochemical analyses were performed on tissue sections and cells in culture.

RESULTS

An RPE-specific epitope, RET-PE10, has been detected as a 61 kD cytoplasmic polypeptide in a variety of mammalian, amphibian, and avian species. In the rat, RPE-PE10 was expressed late in eye development, with a faint initial labelling of the RPE in central regions at postnatal day 9 (PN9) that increased to adult levels and extent of staining by PN14. RET-PE10 expression initially was present in overnight cultures of dissociated rat RPE cells but was lost rapidly from these cultures during the first week. Comparison of the staining patterns of RET-PE10 with those of various cytoskeletal elements suggests that RET-PE10 may be associated with part of the intermediate filament network. Culture of whole eyecups also resulted in a loss of RET-PE10 expression. RET-PE10 expression was normal in eyes of adult rd/rd mutant mice.

CONCLUSIONS

RET-PE10 is a late-appearing marker of RPE differentiation. The results also suggest that the maintained expression of RET-PE10 depends upon extrinsic factors but that these do not include maintained contact with Bruch's membrane or light-induced retinal activity.

Renal epithelial cells show nonselective cation channel activity and express a gene related to the cGMP-gated photoreceptor channel

Nonselective cation channels have been found in various parts of the nephron and represent a heterogeneous group of channels. We briefly review their putative physiological function. Renal epithelial nonselective cation channels may play a role in volume regulation, calcium entry, cell proliferation, and sodium reabsorption. In some renal epithelia cGMP seems to be involved in the regulation of nonselective cation channels. Furthermore, there is evidence that a gene related to the cGMP-gated photoreceptor channel, a well-characterized, nonselective cation channel, is also expressed in whole rat kidney tissue. In the context of these observations, we review recent findings from our own work on a nonselective cation channel in the M-1 mouse cortical collecting duct cell line. We could demonstrate that M-1 cells show nonselective cation channel activity in inside-out patches and express a gene related to the cGMP-gated photoreceptor channel (Proc. Natl. Acad. Sci. USA 89:10262-10266, 1992). The possibility of a relation between the kidney channel and the photoreceptor channel is discussed.

Differential laminar expression of particulate and soluble guanylate cyclase genes in rat retina

Expression of both particulate and soluble forms of guanylate cyclase have been identified in adult rat retina using reverse transcriptase-PCR amplification of retinal RNA and sequencing of the cloned cDNAs. Over a 267-amino acid region, the retinal particulate guanylate cyclase was found to be identical in sequence to the GC-A form of the enzyme found in many tissues. No expression of the closely related GC-B or GC alpha forms of the enzyme was found. mRNA corresponding to both subunits of the soluble form of guanylate cyclase were detected in retinal. The sequence corresponding to the 70-kDa subunit was identical to that from rat lung over a 304-amino acid region and the sequence corresponding to the 82-kDa subunit showed only one amino acid difference over a 275-amino acid region. From Northern analyses the level of expression of the soluble guanylate cyclase in retina was higher than that in lung. In situ hybridization to sections of adult retina indicated that the particulate guanylate cyclase was expressed predominantly in rod photoreceptors. Although the soluble form of the enzyme was detected in all retinal layers, the level of expression was much higher in the inner nuclear layer. The results suggest that multiple enzymes and hence multiple regulatory pathways may control cGMP levels in rod photoreceptors and that cGMP may play an important role in neurons of the inner retina.

Mouse cortical collecting duct cells show nonselective cation channel activity and express a gene related to the cGMP-gated rod photoreceptor channel

Apical nonselective cation channels with an average single-channel conductance of 34 +/- 2.3 pS were found in M-1 mouse cortical collecting duct cells. Channel activity is increased by depolarization and abolished by cytoplasmic calcium removal. Cytoplasmic application of 0.1 mM cGMP decreases channel open probability by 27%. cDNAs corresponding to approximately 40% of the coding region of the photoreceptor channel were isolated by the polymerase chain reaction from M-1 cells and a rat kidney cDNA library. The rat kidney-derived sequence differs by a single base, and the M-1-cell-derived sequence differs by only two bases, from the photoreceptor sequence. A second clone from M-1 cells differs by 20 out of 426 bases from the photoreceptor sequence. In all three clones, the deduced amino acid sequence is identical to that of the rat photoreceptor channel. Northern blot analysis of poly(A)+ RNA from M-1 cells reveals the presence of a 3.2-kilobase band hybridizing with a retinal cGMP-gated cation channel probe. The results suggest the expression in M-1 cells of more than one gene coding for nonselective cation channels or channel subunits, one of which is identical to the cGMP-gated cation channel gene of rod photoreceptors.