Growth factors regulate phototransduction in retinal rods by modulating cyclic nucleotide-gated channels through dephosphorylation of a specific tyrosine residue

The neurobiology of insulin-like growth factor I: From neuroprotection to modulation of brain states

After decades of research in the neurobiology of IGF-I, its role as a prototypical neurotrophic factor is undisputed. However, many of its actions in the adult brain indicate that this growth factor is not only involved in brain development or in the response to injury. Following a three-layer assessment of its role in the central nervous system, we consider that at the cellular level, IGF-I is indeed a bona fide neurotrophic factor, modulating along ontogeny the generation and function of all the major types of brain cells, contributing to sculpt brain architecture and adaptive responses to damage. At the circuit level, IGF-I modulates neuronal excitability and synaptic plasticity at multiple sites, whereas at the system level, IGF-I intervenes in energy allocation, proteostasis, circadian cycles, mood, and cognition. Local and peripheral sources of brain IGF-I input contribute to a spatially restricted, compartmentalized, and timed modulation of brain activity. To better define these variety of actions, we consider IGF-I a modulator of brain states. This definition aims to reconcile all aspects of IGF-I neurobiology, and may provide a new conceptual framework in the design of future research on the actions of this multitasking neuromodulator in the brain.

Adaptation memory in photoreceptors: different mechanisms in rods and cones

Vertebrate rods and cones operate over a wide range of ambient illumination, which is provided by light adaptation mechanisms regulating the sensitivity and speed of the phototransduction cascade. Three calcium-sensitive feedback loops are well established in both rods and cones: acceleration of the quenching of a light-activated visual pigment and cGMP synthesis by guanylate cyclase, and increased affinity of ion channels for cGMP. Accumulating evidence suggests that the molecular mechanisms of light adaptation are more complex. While investigating these putative mechanisms, we discovered a novel phenomenon, observing that the recovery of light sensitivity in rods after turning off non-saturating adaptive light can take tens of seconds. Moreover, after a formal return of the membrane current to the dark level, cell sensitivity to the stimuli remains decreased for a further 1-2 min. We termed this phenomenon of prolonged photoreceptor desensitization 'adaptation memory' (of previous illumination) and the current study is focused on its detailed investigation in rods and an attempt to find the same phenomenon in cones. In rods, we have explored the dependencies of this phenomenon on adapting conditions, specifically, the intensity and duration of adapting illumination. Additionally, we report that fish and frog red-sensitive cones possess similar features of adaptation memory, such as a drop in sensitivity just after the steps of bright light and slow sensitivity recovery. However, we have found that the rate of this process and its nature are not the same as in rods. Our results indicate that the nature of the temporary drop in the sensitivity in rods and cones after adapting steps of light is different. In the rods, adaptation memory could be attributed to the existence of long-lasting modifications of the components of the phototransduction cascade after adapting illumination. In cones, the observed form of the adaptation memory seems to be due to the sensitivity drop caused by a decrease in the availability of the visual pigment, that is, by bleaching.

IGF-1 facilitates extinction of conditioned fear

Insulin-like growth factor-1 (IGF-1) plays a key role in synaptic plasticity, spatial learning, and anxiety-like behavioral processes. While IGF-1 regulates neuronal firing and synaptic transmission in many areas of the central nervous system, its signaling and consequences on excitability, synaptic plasticity, and animal behavior dependent on the prefrontal cortex remain unexplored. Here, we show that IGF-1 induces a long-lasting depression of the medium and slow post-spike afterhyperpolarization (mAHP and sAHP), increasing the excitability of layer 5 pyramidal neurons of the rat infralimbic cortex. Besides, IGF-1 mediates a presynaptic long-term depression of both inhibitory and excitatory synaptic transmission in these neurons. The net effect of this IGF-1-mediated synaptic plasticity is a long-term potentiation of the postsynaptic potentials. Moreover, we demonstrate that IGF-1 favors the fear extinction memory. These results show novel functional consequences of IGF-1 signaling, revealing IGF-1 as a key element in the control of the fear extinction memory.

Role of recoverin in rod photoreceptor light adaptation

KEY POINTS

Recoverin is a small molecular-weight, calcium-binding protein in rod outer segments that can modulate the rate of rhodopsin phosphorylation. We describe two additional and perhaps more important functions during photoreceptor light adaptation. Recoverin influences the rate of change of adaptation. In wild-type rods, sensitivity and response integration time adapt with similar time constants of 150-200 ms. In Rv-/- rods lacking recoverin, sensitivity declines faster and integration time is already shorter and not significantly altered. During steady light exposure, rod circulating current slowly increases during a time course of tens of seconds, gradually extending the operating range of the rod. In Rv-/- rods, this mechanism is deleted, steady-state currents are already larger and rods saturate at brighter intensities. We propose that recoverin modulates spontaneous and light-activated phophodiesterase-6, the phototransduction effector enzyme, to increase sensitivity in dim light but improve responsiveness to change in brighter illumination.

ABSTRACT

Recoverin is a small molecular-weight, calcium-binding protein in rod outer segments that binds to G-protein receptor kinase 1 and can alter the rate of rhodopsin phosphorylation. A change in phosphorylation should change the lifetime of light-activated rhodopsin and the gain of phototransduction, but deletion of recoverin has little effect on the sensitivity of rods either in the dark or in dim-to-moderate background light. We describe two additional functions perhaps of greater physiological significance. (i) When the ambient intensity increases, sensitivity and integration time decrease in wild-type (WT) rods with similar time constants of 150-200 ms. Recoverin is part of the mechanism controlling this process because, in Rv-/- rods lacking recoverin, sensitivity declines more rapidly and integration time is already shorter and not further altered. (ii) During steady light exposure, WT rod circulating current slowly increases during a time course of tens of seconds, gradually extending the operating range of the rod. In Rv-/- rods, this mechanism is also deleted, steady-state currents are already larger and rods saturate at brighter intensities. We argue that neither (i) nor (ii) can be caused by modulation of rhodopsin phosphorylation but may instead be produced by direct modulation of phophodiesterase-6 (PDE6), the phototransduction effector enzyme. We propose that recoverin in dark-adapted rods keeps the integration time long and the spontaneous PDE6 rate relatively high to improve sensitivity. In background light, the integration time is decreased to facilitate detection of change and motion and the spontaneous PDE6 rate decreases to augment the rod working range.

Structural Basis for Interactions Between Contactin Family Members and Protein-tyrosine Phosphatase Receptor Type G in Neural Tissues

Protein-tyrosine phosphatase receptor type G (RPTPγ/PTPRG) interacts in vitro with contactin-3-6 (CNTN3-6), a group of glycophosphatidylinositol-anchored cell adhesion molecules involved in the wiring of the nervous system. In addition to PTPRG, CNTNs associate with multiple transmembrane proteins and signal inside the cell via cis-binding partners to alleviate the absence of an intracellular region. Here, we use comprehensive biochemical and structural analyses to demonstrate that PTPRG·CNTN3-6 complexes share similar binding affinities and a conserved arrangement. Furthermore, as a first step to identifying PTPRG·CNTN complexes in vivo, we found that PTPRG and CNTN3 associate in the outer segments of mouse rod photoreceptor cells. In particular, PTPRG and CNTN3 form cis-complexes at the surface of photoreceptors yet interact in trans when expressed on the surfaces of apposing cells. Further structural analyses suggest that all CNTN ectodomains adopt a bent conformation and might lie parallel to the cell surface to accommodate these cis and trans binding modes. Taken together, these studies identify a PTPRG·CNTN complex in vivo and provide novel insights into PTPRG- and CNTN-mediated signaling.

Signalling beyond photon absorption: extracellular retinoids and growth factors modulate rod photoreceptor sensitivity

KEY POINTS

We propose that the end product of chromophore bleaching in rod photoreceptors, all-trans retinol, is part of a feedback loop that increases the sensitivity of the phototransduction cascade in rods. A previously described light-induced hypersensitivity in rods, termed adaptive potentiation, is reduced by exogenously applied all-trans retinol but not all-trans retinal. This potentiation is produced by insulin-like growth factor-1, whose binding proteins are located in the extracellular matrix, even in our isolated retina preparation after removal of the retinal pigmented epithelium. Simple modelling suggests that the light stimuli used in the present study will produce sufficient all-trans retinol within the interphotoreceptor matrix to explain the potentiation effect.

ABSTRACT

Photoreceptors translate the absorption of photons into electrical signals for propagation through the visual system. Mammalian photoreceptor signalling has largely been studied in isolated cells, and such studies have necessarily avoided the complex environment of supportive proteins that surround the photoreceptors. The interphotoreceptor matrix (IPM) contains an array of proteins that aid in both structural maintenance and cellular homeostasis, including chromophore turnover. In signalling photon absorption, the chromophore 11-cis retinal is first isomerized to all-trans retinal, followed by conversion to all-trans retinol (ROL) for removal from the photoreceptor. Interphotoreceptor retinoid-binding protein (IRBP) is the most abundant protein in the IPM, and it promotes the removal of bleached chromophores and recycling in the nearby retinal pigment epithelium. By studying the light responses of isolated mouse retinas, we demonstrate that ROL can act as a feedback signal onto photoreceptors that influences the sensitivity of phototransduction. In addition to IRBP, the IPM also contains insulin-like growth factor-1 (IGF-1) and its associated binding proteins, although their functions have not yet been described. We demonstrate that extracellular application of physiological concentrations of IGF-1 can increase rod photoreceptor sensitivity in mammalian retinas. We also determine that chromophores and growth factors can limit the range of a newly described form of photoreceptor light adaptation. Finally, fluorescent antibodies demonstrate the presence of IRBP and IGFBP-3 in isolated retinas. A simple model of the formation and release of ROL into the extracellular space quantitatively describes this novel feedback loop.

Adaptive potentiation in rod photoreceptors after light exposure

Photoreceptors adapt to changes in illumination by altering transduction kinetics and sensitivity, thereby extending their working range. We describe a previously unknown form of rod photoreceptor adaptation in wild-type (WT) mice that manifests as a potentiation of the light response after periods of conditioning light exposure. We characterize the stimulus conditions that evoke this graded hypersensitivity and examine the molecular mechanisms of adaptation underlying the phenomenon. After exposure to periods of saturating illumination, rods show a 10-35% increase in circulating dark current, an adaptive potentiation (AP) to light exposure. This potentiation grows as exposure to light is extended up to 3 min and decreases with longer exposures. Cells return to their initial dark-adapted sensitivity with a time constant of recovery of ∼7 s. Halving the extracellular Mg concentration prolongs the adaptation, increasing the time constant of recovery to 13.3 s, but does not affect the magnitude of potentiation. In rods lacking guanylate cyclase activating proteins 1 and 2 (GCAP(-/-)), AP is more than doubled compared with WT rods, and halving the extracellular Mg concentration does not affect the recovery time constant. Rods from a mouse expressing cyclic nucleotide-gated channels incapable of binding calmodulin also showed a marked increase in the amplitude of AP. Application of an insulin-like growth factor-1 receptor (IGF-1R) kinase inhibitor (Tyrphostin AG1024) blocked AP, whereas application of an insulin receptor kinase inhibitor (HNMPA(AM)3) failed to do so. A broad-acting tyrosine phosphatase inhibitor (orthovanadate) also blocked AP. Our findings identify a unique form of adaptation in photoreceptors, so that they show transient hypersensitivity to light, and are consistent with a model in which light history, acting via the IGF-1R, can increase the sensitivity of rod photoreceptors, whereas the photocurrent overshoot is regulated by Ca-calmodulin and Ca(2+)/Mg(2+)-sensitive GCAPs.

Trophic factors in the pathogenesis and therapy for retinal degenerative diseases

Trophic factors are endogenously secreted proteins that act in an autocrine and/or paracrine fashion to affect vital cellular processes such as proliferation, differentiation, and regeneration, thereby maintaining overall cell homeostasis. In the eye, the major contributors of these molecules are the retinal pigment epithelial (RPE) and Müller cells. The primary paracrine targets of these secreted proteins include the photoreceptors and choriocapillaris. Retinal degenerative diseases such as age-related macular degeneration and retinitis pigmentosa are characterized by aberrant function and/or eventual death of RPE cells, photoreceptors, choriocapillaris, and other retinal cells. We discuss results of in vitro and in vivo animal studies in which candidate trophic factors, either singly or in combination, were used in an attempt to ameliorate photoreceptor and/or retinal degeneration. We also examine current trophic factor therapies as they relate to the treatment of retinal degenerative diseases in clinical studies.

Insulin receptor regulates photoreceptor CNG channel activity

Photoreceptor cyclic nucleotide gated (CNG) channels are critical elements in phototransduction and light adaptation. Here we report that insulin receptor (IR), an integral membrane protein, directly phosphorylates the CNGA1 subunit of CNG channels that in turn affects the function of these channels negatively. The IR phosphorylates Tyr(498) and Tyr(503) residues on CNGA1 that are situated at the membrane-cytoplasmic interface. The IR tyrosine kinase activity is essential for the inhibition of CNG channel. To maintain the channels in an off state, it is necessary not only to have a precise balance of the cGMP levels but also to have a control on the cGMP sensitivity of the CNG channels itself. In this study, we observed that the channel opens at a lower concentration of cGMP in IR(-/-) mice. These studies suggest that IR regulates the modulation of CNG channel activity in vivo.

TC-PTP dephosphorylates the guanine nucleotide exchange factor C3G (RapGEF1) and negatively regulates differentiation of human neuroblastoma cells

The guanine nucleotide exchange factor, C3G (RapGEF1), functions in multiple signaling pathways involved in cell adhesion, proliferation, apoptosis and actin reorganization. C3G is regulated by tyrosine phosphorylation on Y504, known to be mediated by c-Abl and Src family kinases. In the present study we explored the possibility of cellular phospho-C3G (pC3G) being a substrate of the intracellular T-cell protein tyrosine phosphatase TC-PTP (PTPN2) using the human neuroblastoma cell line, IMR-32. In vivo and in vitro binding assays demonstrated interaction between C3G and TC-PTP. Interaction is mediated through the Crk-binding region of C3G and C-terminal noncatalytic residues of TC-PTP. C3G interacted better with a substrate trap mutant of TC48 and this complex formation was inhibited by vanadate. Endogenous pC3G colocalized with catalytically inactive mutant TC48 in the Golgi. Expression of TC48 abrogated pervanadate and c-Src induced phosphorylation of C3G without affecting total cellular phospho-tyrosine. Insulin-like growth factor treatment of c-Src expressing cells resulted in dephosphorylation of C3G dependent on the activity of endogenous TC48. TC48 expression inhibited forskolin induced tyrosine phosphorylation of C3G and neurite outgrowth in IMR-32 cells. Our results identify a novel Golgi localized substrate of TC48 and delineate a role for TC48 in dephosphorylation of substrates required during differentiation of human neuroblastoma cells.

Tyrosine phosphorylation of cGMP-gated ion channels is under circadian control in chick retina photoreceptors

PURPOSE

To investigate the role of tyrosine phosphorylation in circadian regulation of cGMP-gated cation channels (CNGCs) of chicken cone photoreceptors.

METHODS

Chick retinas were studied on the second day of constant darkness (DD) after several days of entrainment to 12:12 hr light-dark (LD) cycles in vitro. Inside-out patch recordings were made during the subjective day and subjective night to quantify circadian changes in the sensitivity of CNGCs to activation by cGMP after treatment with various tyrosine kinase and tyrosine phosphatase inhibitors. Immunoprecipitation and immunoblot analysis were also used to examine tyrosine phosphorylation of CNGCs and closely associated proteins after separation by conventional and two-dimensional SDS-PAGE.

RESULTS

Treatment with tyrosine kinase inhibitors caused a significant decrease in K(1/2) for cGMP activation of CNGCs in patches excised from cones during the subjective day, but had no effect on K(1/2) during the subjective night. Conversely, treatment with a tyrosine phosphatase inhibitor caused a significant increase in the K(1/2) of CNGCs in patches excised during the subjective night but had no effect on channel K(1/2) during the subjective day. Broad spectrum serine-threonine phosphatase inhibitors had no effect. An 85-kDa tyrosine polypeptide that coimmunoprecipitated with CNGC alpha-subunits was detectable at higher levels during the subjective day than during the subjective night. CNGC alpha-subunits were not tyrosine phosphorylated as a function of the time of day.

CONCLUSIONS

Circadian control of cone CNGCs appears to entail elevated daytime tyrosine phosphorylation of an approximately 85-kDa auxiliary protein or another subunit of the CNGCs.

Regulation of human cone cyclic nucleotide-gated channels by endogenous phospholipids and exogenously applied phosphatidylinositol 3,4,5-trisphosphate

Cyclic nucleotide-gated (CNG) channels are critical components of the vertebrate visual transduction cascade involved in converting light-induced changes in intracellular cGMP concentrations into electrical signals that can be interpreted by the brain as visual information. To characterize regulatory mechanisms capable of altering the apparent ligand affinity of cone channels, we have expressed heteromeric (CNGA3 + CNGB3) human cone CNG channels in Xenopus laevis oocytes and characterized the alterations in channel activity that occur after patch excision using patch-clamp recording in the inside-out configuration. We found that cone channels exhibit spontaneous changes in current at subsaturating cGMP concentrations; these changes are enhanced by application of ATP and seem to reflect alterations in channel gating. Similar to rod CNG channels, lavendustin A prevented this regulation, suggesting the involvement of a tyrosine phosphorylation event. However, the tyrosine residue in CNGB3 (Tyr545) that is equivalent to the critical tyrosine residues in rod and olfactory CNG channel subunits does not participate in cone channel regulation. Furthermore, the changes in ligand sensitivity of CNGA3 + CNGB3 channels were prevented by inhibition of phosphatidylinositol 3-kinase (PI3-kinase) using wortmannin or 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002), which suggests that phospholipid metabolism can regulate the channels. Direct application of phosphatidylinositol 3,4,5-trisphosphate (PIP3) to the intracellular face of excised patches also resulted in down-regulation of channel activity. Thus, phospholipid metabolism and exogenously applied PIP3 can modulate heterologously expressed cone CNG channels.

Insulin-like growth factor-1 inhibits adult supraoptic neurons via complementary modulation of mechanoreceptors and glycine receptors

In the CNS, insulin-like growth factor-1 (IGF-1) is mainly known for its trophic effect both during development and in adulthood. Here, we show than in adult rat supraoptic nucleus (SON), IGF-1 receptor immunoreactivity is present in neurons, whereas IGF-1 immunoreactivity is found principally in astrocytes and more moderately in neurons. In vivo application of IGF-1 within the SON acutely inhibits the activity of both vasopressin and oxytocin neurons, the two populations of SON neuroendocrine cells. Recordings of acutely isolated SON neurons showed that this inhibition occurs through two rapid and reversible mechanisms, both involving the neuronal IGF-1 receptor but different intracellular messengers. IGF-1 inhibits Gd3+-sensitive and osmosensitive mechanoreceptor cation current via phosphatidylinositol-3 (PI3) kinase activation. IGF-1 also potentiates taurine-activated glycine receptor (GlyR) Cl- currents by increasing the agonist sensitivity through a extremely rapid (within a second) PI3 kinase-independent mechanism. Both mechanoreceptor channels and GlyR, which form the excitatory and inhibitory components of SON neuron osmosensitivity, are active at rest, and their respective inhibition and potentiation will both be inhibitory, leading to strong decrease in neuronal activity. It will be of interest to determine whether IGF-1 is released by neurons, thus participating in an inhibitory autocontrol, or astrocytes, then joining the growing family of glia-to-neuron transmitters that modulate neuronal and synaptic activity. Through the opposite and complementary acute regulation of mechanoreceptors and GlyR, IGF-1 appears as a new important neuromodulator in the adult CNS, participating in the complex integration of neural messages that regulates the level of neuronal excitability.

Effects of insulin-like growth factor 1 on voltage-gated ion channels in cultured rat hippocampal neurons

Insulin-like growth factor 1 (IGF-1) has important functions in the brain, including metabolic, neurotrophic, neuromodulatory, and neuroendocrine actions, and it is also prevents amyloid beta-induced death of hippocampal neurons. However, its functions on the voltage-gated ion channels in hippocampus remain uncertain. In the present study, we investigated the effects of IGF-1 on voltage-gated potassium, sodium, and calcium channels in the cultured rat hippocampal neurons using the whole-cell patch clamp recordings. Following incubation with different doses of IGF-1 for 24 h, a block of the peak transient A-type K+ currents amplitude (IC50: 4.425 ng/ml, Hill coefficient: 0.621) was observed. In addition, after the application of IGF-1, the amplitude of high-voltage activated Ca2+ currents significantly increased but activation kinetics did not significantly alter (V1/2: -33.45 +/- 1.32 mV, k = 6.16 +/- 1.05) compared to control conditions (V1/2: -33.19 +/- 2.28 mV, k = 7.26 +/- 1.71). However, the amplitude of Na+, K+, and low-voltage activated Ca2+ currents was not affected by the application of IGF-1. These data suggest that IGF-1 inhibits transient A-type K+ currents and enhances high-voltage-activated Ca2+ currents, but has no effects on Na+ and low-voltage-activated Ca2+ currents.

A novel mechanism of modulation of hyperpolarization-activated cyclic nucleotide-gated channels by Src kinase

Hyperpolarization-activated cyclic nucleotide-gated channels (HCN1-4) play a crucial role in the regulation of cell excitability. Importantly, they contribute to spontaneous rhythmic activity in brain and heart. HCN channels are principally activated by membrane hyperpolarization and binding of cAMP. Here, we identify tyrosine phosphorylation by Src kinase as another mechanism affecting channel gating. Inhibition of Src by specific blockers slowed down activation kinetics of native and heterologously expressed HCN channels. The same effect on HCN channel activation was observed in cells cotransfected with a dominant-negative Src mutant. Immunoprecipitation demonstrated that Src binds to and phosphorylates native and heterologously expressed HCN2. Src interacts via its SH3 domain with a sequence of HCN2 encompassing part of the C-linker and the cyclic nucleotide binding domain. We identified a highly conserved tyrosine residue in the C-linker of HCN channels (Tyr476 in HCN2) that confers modulation by Src. Replacement of this tyrosine by phenylalanine in HCN2 or HCN4 abolished sensitivity to Src inhibitors. Mass spectrometry confirmed that Tyr476 is phosphorylated by Src. Our results have functional implications for HCN channel gating. Furthermore, they indicate that tyrosine phosphorylation contributes in vivo to the fine tuning of HCN channel activity.

Subunit contributions to phosphorylation-dependent modulation of bovine rod cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels in rod photoreceptors transduce a decrease in cGMP into hyperpolarization during the light response. Insulin-like growth factor-1 (IGF-1) increases light responses by increasing the cGMP sensitivity of CNG channels, an event mediated by a protein tyrosine phosphatase. Native rod CNG channels are heteromultimers, composed of three CNGA1 subunits and one CNGB1 subunit. Previous studies on heterologously expressed rod CNG channels show that a specific tyrosine in the CNGA1 subunit (Y498) is required for modulation by protein tyrosine phosphatases, protein tyrosine kinases and IGF-1. Here we show that the CNGB1 subunit contains a specific tyrosine (Y1097) that is important for modulation of heteromeric channels by tyrosine phosphorylation. Direct biochemical measurements demonstrate 32P-labelling of CNGA1Y498 and CNGB1Y1097. Replacement of either Y498 of CNGA1 or Y1097 of CNGB1 with phenylalanine reduces modulation, and removal of both tyrosines eliminates modulation. Unlike CNGA1, CNGB1 does not exhibit activity dependence of modulation by tyrosine phosphorylation. Hence both CNGA1 and CNGB1 subunits contribute to phosphorylation-dependent modulation of rod CNG channels, but the phosphorylation states of the two subunits are regulated in different ways.

Regulation of nitric oxide and soluble guanylyl cyclase

Since the discoveries that have verified nitric oxide (NO) as an endogenously produced cell signaling molecule, research surrounding its production and mechanisms of action have been studied at an exponentially increasing rate. NO is produced by a family of enzymes termed the NO synthases (NOS), which are regulated independently by various stimuli. Once produced, NO can solicit numerous biological events by reacting with various metals, thiols, and oxygen species to modify proteins, DNA and lipids. One of the most biologically relevant actions of NO is its binding to the heme moiety in the heterodimeric enzyme, soluble guanylyl cyclase (sGC). Activation of sGC by NO results in the production of the second messenger molecule, 3',5'-cyclic guanosine monophosphate (cGMP), which can regulate numerous physiological events such as vasodilatation and neurotransmission. Here we will review the synthesis and fate of NO, and discuss the activation and regulation of the NO receptor, sGC.

Circadian regulation of cGMP-gated channels of vertebrate cone photoreceptors: role of cAMP and Ras

Circadian oscillators in chicken cone photoreceptors regulate the gating properties of cGMP-gated cationic channels (CNGCs) such that they have a higher apparent affinity for cGMP during the subjective night. Here we show that cAMP, acting through protein kinase A (PKA), Ras, and Erk, is part of the circadian output pathway controlling CNGCs. Endogenous and exogenous cAMP cause activation of Erk and Ras, which are more active at night in cones, and increase the apparent affinity of CNGCs for cGMP. The Ras farnesyl transferase inhibitor manumycin-A, and a dominant-negative form of Ras (RasN17) block the circadian rhythms in CNGC gating, as well as the effects of cAMP. A dominant-negative form of the MEK kinase B-Raf also blocks circadian and cAMP modulation of CNGCs. The circadian output pathway modulating CNGC channels is comprised in part of cAMP --> PKA --> Ras --> B-Raf --> MEK --> Erk --> --> CNGCs. cAMP activation of Ras and Erk occur within minutes, whereas modulation of CNGCs requires >1 hr. However, cAMP protagonists do not alter rhythms in cPer2 mRNA, and their effects on CNGCs cannot be attributed to clock phase-shifting.

Bleaching desensitization: background and current challenges

"Bleaching desensitization" in rod photoreceptors refers to the prolonged depression of phototransduction sensitivity exhibited by rods after their exposure to bright light, i.e., after photolysis (bleaching) of a substantial fraction of rhodopsin in the outer segments. Rod recovery from bleaching desensitization depends critically on operation of the retinoid visual cycle: in particular, on the removal of all-trans retinal bleaching product from opsin and on the delivery of 11-cis retinal to opsin's chromophore binding site. The present paper summarizes representative findings that address the mechanism of bleaching desensitization.

Tyrosine phosphorylation of rod cyclic nucleotide-gated channels switches off Ca2+/calmodulin inhibition

Cyclic nucleotide-gated (CNG) ion channels are crucial for phototransduction in rod photoreceptors. Light triggers a biochemical cascade that reduces the concentration of cGMP in rods, closing CNG channels, which leads to membrane potential hyperpolarization and a decrease in the concentration of intracellular Ca2+. During light adaptation, the sensitivity of CNG channels to cGMP is decreased by Ca2+, which in conjunction with calmodulin (CaM), binds directly to CNG channels. The cGMP sensitivity of rod CNG channels is also reduced by phosphorylation of specific tyrosine residues in the three CNGA1 subunits and one CNGB1 subunit that comprise the rod channel. Here we show that phosphorylation prevents Ca2+/CaM inhibition. Experiments on native channels in rod outer segments and expressed channels in Xenopus oocytes show that Ca2+/CaM inhibition can be toggled off or on by promoting phosphorylation or dephosphorylation, respectively. Experiments in which the crucial tyrosine phosphorylation sites in CNGA1 and CNGB1 are replaced with phenylalanines show that residue Y498 in CNGA1 is the phosphorylation site responsible for regulating Ca2+/CaM inhibition. Ca2+/CaM inhibits the rod channel by binding to the N terminus of the CNGB1 subunit, causing it to uncouple from the C terminus of CNGA1. We propose that phosphorylation of CNGA1Y498, on the C terminus of CNGA1, triggers an equivalent uncoupling from the C terminus of CNGB1, thereby curtailing Ca2+/CaM inhibition. The control of CaM inhibition by CNG channel phosphorylation may be important for light adaptation and the regulation of phototransduction by IGF-1, a retinal paracrine factor that alters the tyrosine phosphorylation state of rod CNG channels.

Tyrosine phosphorylation of rod cyclic nucleotide-gated channels switches off Ca2+/calmodulin inhibition

Cyclic nucleotide-gated (CNG) ion channels are crucial for phototransduction in rod photoreceptors. Light triggers a biochemical cascade that reduces the concentration of cGMP in rods, closing CNG channels, which leads to membrane potential hyperpolarization and a decrease in the concentration of intracellular Ca2+. During light adaptation, the sensitivity of CNG channels to cGMP is decreased by Ca2+, which in conjunction with calmodulin (CaM), binds directly to CNG channels. The cGMP sensitivity of rod CNG channels is also reduced by phosphorylation of specific tyrosine residues in the three CNGA1 subunits and one CNGB1 subunit that comprise the rod channel. Here we show that phosphorylation prevents Ca2+/CaM inhibition. Experiments on native channels in rod outer segments and expressed channels in Xenopus oocytes show that Ca2+/CaM inhibition can be toggled off or on by promoting phosphorylation or dephosphorylation, respectively. Experiments in which the crucial tyrosine phosphorylation sites in CNGA1 and CNGB1 are replaced with phenylalanines show that residue Y498 in CNGA1 is the phosphorylation site responsible for regulating Ca2+/CaM inhibition. Ca2+/CaM inhibits the rod channel by binding to the N terminus of the CNGB1 subunit, causing it to uncouple from the C terminus of CNGA1. We propose that phosphorylation of CNGA1Y498, on the C terminus of CNGA1, triggers an equivalent uncoupling from the C terminus of CNGB1, thereby curtailing Ca2+/CaM inhibition. The control of CaM inhibition by CNG channel phosphorylation may be important for light adaptation and the regulation of phototransduction by IGF-1, a retinal paracrine factor that alters the tyrosine phosphorylation state of rod CNG channels.

Regulation of the rod photoreceptor cyclic nucleotide-gated channel

The cGMP-gated channel of rod photoreceptors plays a key role in phototransduction by controlling the flow of cations into the outer segment in response to light-induced changes in cGMP. The channel is a heterotetramer composed of alpha-subunits required for channel activity and beta-subunits that are important in modulating the activity of channel. Earlier studies have shown that exogenous calmodulin binds to the beta-subunit of the channel and modulates the sensitivity of the channel for cGMP in a calcium dependent manner. In addition unidentified Ca2+-dependent endogenous proteins have been reported to modulate the activity of the frog rod channel. In this paper, we investigated whether endogenous calmodulin and other Ca2+ binding proteins interact with and modulate the cGMP-gated channel in bovine rod outer segments. Using immunoaffinity techniques in conjunction with ion flux assays, we show that endogenous calmodulin, but not other Ca2+ dependent proteins, binds and modulates the rod cGMP-gated channel in bovine rod outer segments. We also show that the beta-subunit of the channel is phosphorylated by endogenous and exogenous casein kinase 2. This posttranslational modification, however, does not alter the sensitivity of the channel for cGMP.

Insulin-like growth factor I modifies electrophysiological properties of rat brain stem neurons

On systemic injection, insulin-like growth factor I (IGF-I) elicits a prolonged increase in the excitability of dorsal column nuclei (DCN) cells in the brain stem as well as other target neurons within the brain. We have explored the cellular mechanisms involved in the stimulatory effects of IGF-I as well as its functional consequences. In a rat slice preparation, IGF-I induced a sustained depolarization of 2-5 mV in 81% of DCN neurons. Depolarization was accompanied with an increase in the input resistance (15%). Voltage-clamp recordings displayed that IGF-I decreased a K+-mediated A current (60%). Furthermore, IGF-I increased, in 78% of cells, the peak amplitude (25%), and rising slope (32%) of the excitatory postsynaptic potential evoked by dorsal column stimulation; in this case, a presynaptic facilitatory process appears to be involved. When anesthetized adult rats are injected in the carotid artery with IGF-I, extracellularly recorded propioceptive DCN neurons not only show increased spike activity but also an expansion of their cutaneous receptive field in 83% of DCN cells. Significantly, the increased excitability evoked by IGF-I in the DCN cells depends both in vivo and in vitro, on activation of p38 mitogen-activated protein kinase (MAPK), a Ser-kinase known to modulate K+ channel activity. We concluded that systemic IGF-I modulated the electrophysiological properties of target neurons within the brain. In turn, these changes probably contribute to functional reorganization processes such as expansion of neuronal receptive fields.

Brain repair and neuroprotection by serum insulin-like growth factor I

The existence of protective mechanisms in the adult brain is gradually being recognized as an important aspect of brain function. For many years, self-repair processes in the post-embryonic brain were considered of minor consequence or nonexistent. This notion dominated the study of neurotrophism. Thus, although the possibility that neurotrophic factors participate in brain function in adult life was prudently maintained, the majority of the studies on the role of trophic factors in the brain were focused on developmental aspects. With the recent recognition that the adult brain keeps a capacity for cell renewal, although limited, a new interest in the regenerative properties of brain tissue has emerged. New findings on the role of insulin-like growth factor I (IGF-I), a potent neurotrophic peptide present at high levels in serum, may illustrate this current trend. Circulating IGF-I is an important determinant of proper brain function in the adult. Its pleiotropic effects range from classical trophic actions on neurons such as housekeeping or anti-apoptotic/ pro-survival effects to modulation of brain-barrier permeability, neuronal excitability, or new neuron formation. More recent findings indicate that IGF-I participates in physiologically relevant neuroprotective mechanisms such as those triggered by physical exercise. The increasing number of neurotrophic features displayed by serum IGF-I reinforces the view of a physiological neuroprotective network formed by IGF-I, and possibly other still uncharacterized signals. Future studies with IGF-I, and hopefully other neurotrophic factors, will surely reveal and teach us how to potentiate the self-reparative properties of the adult brain.

Potentiation of 'on' bipolar cell flash responses by dim background light and cGMP in dogfish retinal slices

The high sensitivity of the vertebrate visual system results from amplification inherent in phototransduction in rods and from the amplification of rod signals on their synaptic transfer at the first synapse with 'on' bipolar cells. These cells possess a metabotropic glutamate receptor linked via a cGMP cascade to the control of cGMP-activated channels. In the study presented here, we show that very dim background light, isomerising only one rhodopsin in 1 out of 10 rods per second, potentiates 'on' bipolar cell responses to superimposed flashes. Responses to dim flashes, which were undetectable above the noise in the dark, were boosted above the increased noise level induced by the background. This potentiation could be reproduced by elevating cGMP, which increases with light, or by dialysing the cells with a non-hydrolysable cGMP analogue. Inhibition of tyrosine kinase activity also reproduced the effect and induced a speeding up of the rising phase of the flash response, similar to the action of dim background light. Conversely, inhibition of tyrosine phosphatase activity blocked the potentiation. These results suggest that cGMP promotes tyrosine-site dephosphorylation of 'on' bipolar cell cGMP-activated channels, resulting in a rise in the sensitivity to cGMP, as has recently been demonstrated for rod cGMP-activated channels. This constitutes a positive feedback mechanism such that as cGMP increases with light, the sensitivity of the channels to cGMP increases and boosts the signal above background noise. This mechanism would allow stochastic resonance to occur, facilitating single-photon detection when dark-adapted, and may therefore lead to improved discrimination.

Regulated cationic channel function in Xenopus oocytes expressing Drosophila big brain

Big brain (bib) is a neurogenic gene that when mutated causes defects in cell fate determination during Drosophila neurogenesis through an unknown mechanism. The protein Big Brain (BIB) has sequence identity with the major intrinsic protein family that includes the water- and ion-conducting aquaporin channels. We show here that BIB expressed heterologously in Xenopus oocytes provides a voltage-insensitive, nonselective cation channel function with permeability to K+ > Na+ >> tetraethylammonium. The conductance, activated in response to endogenous signaling pathways in BIB-expressing oocytes, is decreased after treatment with 20 microm insulin and is enhanced with 10 microm lavendustin A, a tyrosine kinase inhibitor. Western blot analysis confirms that BIB is tyrosine-phosphorylated. Both tyrosine phosphorylation and the potentiating effect of lavendustin A are removed by partial deletion of the C terminus (amino acids 317-700). Current activation is not observed in control oocytes or in oocytes expressing a nonfunctional mutant (BIB E71N) that appears to be expressed on the plasma membrane by confocal microscopy and Western blotting. These results indicate that BIB can participate in tyrosine kinase-regulated transmembrane signaling and may suggest a role for membrane depolarization in the neurogenic function of BIB in early development.

Modulation of cyclic-nucleotide-gated channels and regulation of vertebrate phototransduction

Cyclic-nucleotide-gated (CNG) channels are crucial for sensory transduction in the photoreceptors (rods and cones) of the vertebrate retina. Light triggers a decrease in the cytoplasmic concentration of cyclic GMP in the outer segments of these cells, leading to closure of CNG channels and hyperpolarization of the membrane potential. Hence, CNG channels translate a chemical change in cyclic nucleotide concentration into an electrical signal that can spread through the photoreceptor cell and be transmitted to the rest of the visual system. The sensitivity of phototransduction can be altered by exposing the cells to light, through adaptation processes intrinsic to photoreceptors. Intracellular Ca2+ is a major signal in light adaptation and, in conjunction with Ca2+-binding proteins, one of its targets for modulation is the CNG channel itself. However, other intracellular signals may be involved in the fine-tuning of light sensitivity in response to cues internal to organisms. Several intracellular signals are candidates for mediating changes in cyclic GMP sensitivity including transition metals, such as Ni2+ and Zn2+, and lipid metabolites, such as diacylglycerol. Moreover, CNG channels are associated with protein kinases and phosphatases that catalyze changes in phosphorylation state and allosterically modulate channel activity. Recent studies suggest that the effects of circadian rhythms and retinal transmitters on CNG channels may be mediated by such changes in phosphorylation. The goal of this paper is to review the molecular mechanisms underlying modulation of CNG channels and to relate these forms of modulation to the regulation of light sensitivity.