Alternate FGF2-ERK1/2 signaling pathways in retinal photoreceptor and glial cells in vitro

Modeling inducible neuropathologies of the retina with differential phenotypes in organoids

Neurodegenerative diseases remain incompletely understood and therapies are needed. Stem cell-derived organoid models facilitate fundamental and translational medicine research. However, to which extent differential neuronal and glial pathologic processes can be reproduced in current systems is still unclear. Here, we tested 16 different chemical, physical, and cell functional manipulations in mouse retina organoids to further explore this. Some of the treatments induce differential phenotypes, indicating that organoids are competent to reproduce distinct pathologic processes. Notably, mouse retina organoids even reproduce a complex pathology phenotype with combined photoreceptor neurodegeneration and glial pathologies upon combined (not single) application of HBEGF and TNF, two factors previously associated with neurodegenerative diseases. Pharmacological inhibitors for MAPK signaling completely prevent photoreceptor and glial pathologies, while inhibitors for Rho/ROCK, NFkB, and CDK4 differentially affect them. In conclusion, mouse retina organoids facilitate reproduction of distinct and complex pathologies, mechanistic access, insights for further organoid optimization, and modeling of differential phenotypes for future applications in fundamental and translational medicine research.

Affibody-mediated controlled release of fibroblast growth factor 2

Protein therapeutics possess high target affinity and specificity, yet short residence times, which limit their broad utility. To overcome this challenge, we used affinity interactions to modulate protein release from a hydrogel delivery vehicle thereby prolonging therapeutic availability. Specifically, we designed an affibody-modified hyaluronan (HA)-based hydrogel as a delivery platform for fibroblast growth factor 2 (FGF2), a neuroprotective and neuroregenerative factor in the central nervous system (CNS). We identified a highly specific affibody binding partner with moderate affinity for FGF2 using yeast surface display and flow cytometry-based screening. Importantly, we demonstrated controlled release of bioactive FGF2 from the hydrogel by varying the ratio of affibody to protein and showed increased thermal stability of FGF2 in the presence of affibody. This versatile delivery platform will allow the distinct, simultaneous release of multiple proteins based on specific affinity interactions.

MITOGEN-ACTIVATED PROTEIN INHIBITORS: FULL-FIELD ELECTRORETINOGRAM DEMONSTRATING GENERALIZED RETINAL DYSFUNCTION

PURPOSE

To report a patient with generalized retinal toxicity to mitogen-activated protein inhibitors.

METHODS

Retrospective case report.

RESULTS

Full-field electroretinogram findings indicate a generalized toxicity to the use of the mitogen-activated protein inhibitor trametinib. There was an improved response and resolution of serous detachments after decreasing the dose.

CONCLUSION

Mitogen-activated protein inhibitors may affect global retinal function, as opposed to the serous detachments that are concentrated in the posterior pole. This may be of importance in further understanding the underlying pathologic mechanisms.

Chronic Activation of Gp1 mGluRs Leads to Distinct Refinement of Neural Network Activity through Non-Canonical p53 and Akt Signaling

Group 1 metabotropic glutamate receptors (Gp1 mGluRs), including mGluR1 and mGluR5, are critical regulators for neuronal and synaptic plasticity. Dysregulated Gp1 mGluR signaling is observed with various neurologic disorders, including Alzheimer’s disease, Parkinson’s disease, epilepsy, and autism spectrum disorders (ASDs). It is well established that acute activation of Gp1 mGluRs leads to elevation of neuronal intrinsic excitability and long-term synaptic depression. However, it remains unknown how chronic activation of Gp1 mGluRs can affect neural activity and what molecular mechanisms might be involved. In the current study, we employed a multielectrode array (MEA) recording system to evaluate neural network activity of primary mouse cortical neuron cultures. We demonstrated that chronic activation of Gp1 mGluRs leads to elevation of spontaneous spike frequency while burst activity and cross-electrode synchronization are maintained at the baseline. We further showed that these neural network properties are achieved through proteasomal degradation of Akt that is dependent on the tumor suppressor p53. Genetically knocking down p53 disrupts the elevation of spontaneous spike frequency and alters the burst activity and cross-electrode synchronization following chronic activation of Gp1 mGluRs. Importantly, these deficits can be restored by pharmacologically inhibiting Akt to mimic inactivation of Akt mediated by p53. Together, our findings reveal the effects of chronic activation of Gp1 mGluRs on neural network activity and identify a unique signaling pathway involving p53 and Akt for these effects. Our data can provide insights into constitutively active Gp1 mGluR signaling observed in many neurologic and psychiatric disorders.

Novel roles of ER stress in repressing neural activity and seizures through Mdm2- and p53-dependent protein translation

Seizures can induce endoplasmic reticulum (ER) stress, and sustained ER stress contributes to neuronal death after epileptic seizures. Despite the recent debate on whether inhibiting ER stress can reduce neuronal death after seizures, whether and how ER stress impacts neural activity and seizures remain unclear. In this study, we discovered that the acute ER stress response functions to repress neural activity through a protein translation-dependent mechanism. We found that inducing ER stress promotes the expression and distribution of murine double minute-2 (Mdm2) in the nucleus, leading to ubiquitination and down-regulation of the tumor suppressor p53. Reduction of p53 subsequently maintains protein translation, before the onset of translational repression seen during the latter phase of the ER stress response. Disruption of Mdm2 in an Mdm2 conditional knockdown (cKD) mouse model impairs ER stress-induced p53 down-regulation, protein translation, and reduction of neural activity and seizure severity. Importantly, these defects in Mdm2 cKD mice were restored by both pharmacological and genetic inhibition of p53 to mimic the inactivation of p53 seen during ER stress. Altogether, our study uncovered a novel mechanism by which neurons respond to acute ER stress. Further, this mechanism plays a beneficial role in reducing neural activity and seizure severity. These findings caution against inhibition of ER stress as a neuroprotective strategy for seizures, epilepsies, and other pathological conditions associated with excessive neural activity.

One-third of epilepsy patients respond poorly to current anti-epileptic drugs. Thus, there is an urgent need to characterize cellular behavior during seizures, and the corresponding molecular mechanisms in order to develop better therapies. Seizures are known to induce ER stress but how the ER stress response functions to modulate seizure activity is unknown. Our study provides evidence to demonstrate a novel and beneficial role for the ER stress response in reducing neural activity and seizure severity. Mechanistically, we found that these beneficial effects are mediated by elevated protein translation, which is triggered by the activation of Mdm2-p53 signaling, during the early ER stress response. Our findings suggest that therapeutic attempts to reduce ER stress in epilepsies may result in worsening seizure activity and therefore caution against inhibition of ER stress as a neuroprotective strategy for epilepsies.

Mdm2 mediates FMRP- and Gp1 mGluR-dependent protein translation and neural network activity

Activating Group 1 (Gp1) metabotropic glutamate receptors (mGluRs), including mGluR1 and mGluR5, elicits translation-dependent neural plasticity mechanisms that are crucial to animal behavior and circuit development. Dysregulated Gp1 mGluR signaling has been observed in numerous neurological and psychiatric disorders. However, the molecular pathways underlying Gp1 mGluR-dependent plasticity mechanisms are complex and have been elusive. In this study, we identified a novel mechanism through which Gp1 mGluR mediates protein translation and neural plasticity. Using a multi-electrode array (MEA) recording system, we showed that activating Gp1 mGluR elevates neural network activity, as demonstrated by increased spontaneous spike frequency and burst activity. Importantly, we validated that elevating neural network activity requires protein translation and is dependent on fragile X mental retardation protein (FMRP), the protein that is deficient in the most common inherited form of mental retardation and autism, fragile X syndrome (FXS). In an effort to determine the mechanism by which FMRP mediates protein translation and neural network activity, we demonstrated that a ubiquitin E3 ligase, murine double minute-2 (Mdm2), is required for Gp1 mGluR-induced translation and neural network activity. Our data showed that Mdm2 acts as a translation suppressor, and FMRP is required for its ubiquitination and down-regulation upon Gp1 mGluR activation. These data revealed a novel mechanism by which Gp1 mGluR and FMRP mediate protein translation and neural network activity, potentially through de-repressing Mdm2. Our results also introduce an alternative way for understanding altered protein translation and brain circuit excitability associated with Gp1 mGluR in neurological diseases such as FXS.

Regulation of fibroblast growth factor 8 (FGF8) in chicken embryonic stem cells differentiation into spermatogonial stem cells

Fibroblast growth factors (FGFs) are essential in regulating the formation of spermatogonial stem cells (SSCs). Here, we explored the effect of FGF8 on chicken SSCs formation by knockdown or overexpression of FGF8 in chicken embryonic stem cells (ESCs) both in vitro and in vivo. Our results showed that knockdown of FGF8 could facilitate the differentiation of ESCs into SSCs, overexpression of FGF8 could promote PGCs self-renewal, inhibit SSCs formation. This study further revealed the positive correlation between the expression level of FGF8 and MAPK/ERK signal. In the absence of FGF8, the expression of downstream genes such as FGFR2, GRB2, RAS, BRAF, RAF1, and MEK2 was not maintained, while overexpressing FGF8 enhances them. Thus, our study demonstrated that FGF8 can regulate germ cell fate by modulating the dynamic equilibrium between differentiation and self-renewal, which provides a new idea for the study of germ cell regulatory network.

Light, lipids and photoreceptor survival: live or let die?

Due to its constant exposure to light and its high oxygen consumption the retina is highly sensitive to oxidative damage, which is a common factor in inducing the death of photoreceptors after light damage or in inherited retinal degenerations. The high content of docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, has been suggested to contribute to this sensitivity. DHA is crucial for developing and preserving normal visual function. However, further roles of DHA in the retina are still controversial. Current data support that it can tilt the scale either towards degeneration or survival of retinal cells. DHA peroxidation products can be deleterious to the retina and might lead to retinal degeneration. However, DHA has also been shown to act as, or to be the source of, a survival molecule that protects photoreceptors and retinal pigment epithelium cells from oxidative damage. We have established that DHA protects photoreceptors from oxidative stress-induced apoptosis and promotes their differentiation in vitro. DHA activates the retinoid X receptor (RXR) and the ERK/MAPK pathway, thus regulating the expression of anti and pro-apoptotic proteins. It also orchestrates a diversity of signaling pathways, modulating enzymatic pathways that control the sphingolipid metabolism and activate antioxidant defense mechanisms to promote photoreceptor survival and development. A deeper comprehension of DHA signaling pathways and context-dependent behavior is required to understand its dual functions in retinal physiology.

Regulation of rod photoreceptor differentiation by STAT3 is controlled by a tyrosine phosphatase

Signal pathways that reduce the levels of tyrosine-phosphorylated STAT3 (pSTAT3) allow late retinal progenitors to exit the cell cycle and enter a terminal differentiation pathway into rod photoreceptors. In the mouse retina, we previously identified PKC-β1 and PKC-γ isoforms as essential components of a key signal pathway and IGF-1 as a major extrinsic factor regulating rod formation. In this manuscript, we demonstrate that PKC decreases phosphotyrosine but not phosphoserine on STAT3 in neonatal mouse retinas. Neither IGF-1 nor PMA induced a significant change in the levels of STAT3 or in the levels of the key proteins regulating STAT3 degradation, SOCS3, and PIAS3. Treatment of neonatal mouse retinal explants with sodium orthovanadate inhibited the PKC-mediated reduction in pSTAT3, indicating a role for a phosphatase. Addition of the PTEN inhibitor bpV(phen) to explant cultures treated with IGF-1 or PMA had no effect on the reduction in pSTAT3 levels, but the effect of both IGF-1 and PMA was blocked by a concentration of the inhibitor NSC87877 that is selective for the phosphatases Shp1 and Shp2. Inhibition of Shp1/2 phosphatases was also sufficient to abolish the IGF1-mediated induction of rod photoreceptor differentiation in the retina explant cultures. We conclude that one or both of these phosphatases are key components regulating the formation of rod photoreceptors in mouse retina.

The ETS transcription factor Etv1 mediates FGF signaling to initiate proneural gene expression during Xenopus laevis retinal development

Fibroblast growth factor signaling plays a significant role in the developing eye, regulating both patterning and neurogenesis. Members of the Pea3/Etv4-subfamily of ETS-domain transcription factors (Etv1, Etv4, and Etv5) are transcriptional activators that are downstream targets of FGF/MAPK signaling, but whether they are required for eye development is unknown. We show that in the developing Xenopus laevis retina, etv1 is transiently expressed at the onset of retinal neurogenesis. We found that etv1 is not required for eye specification, but is required for the expression of atonal-related proneural bHLH transcription factors, and is also required for retinal neuron differentiation. Using transgenic reporters we show that the distal atoh7 enhancer, which is required for the initiation of atoh7 expression in the Xenopus retina, is responsive to both FGF signaling and etv1 expression. Thus, we conclude that Etv1 acts downstream of FGF signaling to regulate the initiation of neurogenesis in the Xenopus retina.

Retinoid X receptor activation is essential for docosahexaenoic acid protection of retina photoreceptors

We have established that docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, promotes survival of rat retina photoreceptors during early development in vitro and upon oxidative stress by activating the ERK/MAPK signaling pathway. Here we have investigated whether DHA turns on this pathway through activation of retinoid X receptors (RXRs) or by inducing tyrosine kinase (Trk) receptor activation. We also evaluated whether DHA release from phospholipids was required for its protective effect. Addition of RXR antagonists (HX531, PA452) to rat retinal neuronal cultures inhibited DHA protection during early development in vitro and upon oxidative stress induced with Paraquat or H2O2. In contrast, the Trk inhibitor K252a did not affect DHA prevention of photoreceptor apoptosis. These results imply that activation of RXRs was required for DHA protection whereas Trk receptors were not involved in this protection. Pretreatment with 4-bromoenol lactone, a phospholipase A2 inhibitor, blocked DHA prevention of oxidative stress-induced apoptosis of photoreceptors. It is noteworthy that RXR agonists (HX630, PA024) also rescued photoreceptors from H2O2-induced apoptosis. These results provide the first evidence that activation of RXRs prevents photoreceptor apoptosis and suggest that DHA is first released from phospholipids and then activates RXRs to promote the survival of photoreceptors.

Bimatoprost protects retinal neuronal damage via Akt pathway

Worldwide, prostaglandin analogs, such as bimatoprost, have become the major therapeutic class for medical treatment of glaucoma because of their efficacy and generally well tolerated systemic safety profile. However, the detailed mechanism of the direct action of bimatoprost on retinal ganglion cells (RGC) has rarely been understood. Thus, in this study, we elucidated the mechanism of the protective effects of bimatoprost on RGC against oxidative stress. To examine the protective effects of bimatoprost, cultured RGC with various concentrations of bimatoprost (in both free acid and amide form) were exposed to l-buthionin-(S,R)-sulfoximine (BSO) plus glutamate or serum depletion in vitro and intravitreal injection of N-methyl-D-aspartate (NMDA) was used to induce retinal damage in vivo. To elucidate the protective mechanism of bimatoprost, we used western blot analysis to investigate the phosphorylation of Akt and extracellular signal-regulated kinase (ERK). Bimatoprost significantly reduced BSO plus glutamate- and serum deprivation-induced death in concentration-dependent manners. Bimatoprost induced activation of Akt and ERK, and a phosphatidylinositol 3-kinase inhibitor, LY294002, attenuated the protective effect of bimatoprost. On the other hand, a mitogen-activated protein kinase kinase inhibitor, U0126, exhibited protective effect unexpectedly. Moreover, ERK was more phosphorylated by attenuation of Akt activity in cultured RGC. In an in vivo study, bimatoprost reduced NMDA-induced RGC death. Taken together, these findings indicate that bimatoprost has protective effects on in vitro and in vivo retinal damage, suggesting that the mechanism underlying may be via the Akt pathway, which may modulate the ERK pathway.

Changes in fibroblast growth factor-2 and FGF receptors in the frog visual system during optic nerve regeneration

We have previously shown that application of fibroblast growth factor-2 (FGF-2) to cut optic nerve axons enhances retinal ganglion cell (RGC) survival in the adult frog visual system. These actions are mediated via activation of its high affinity receptor FGFR1, enhanced BDNF and TrkB expression, increased CREB phosphorylation, and by promoting MAPK and PKA signaling pathways. The role of endogenous FGF-2 in this system is less well understood. In this study, we determine the distribution of FGF-2 and its receptors in normal animals and in animals at different times after optic nerve cut. Immunohistochemistry and Western blot analysis were conducted using specific antibodies against FGF-2 and its receptors in control retinas and optic tecta, and after one, three, and six weeks post nerve injury. FGF-2 was transiently increased in the retina while it was reduced in the optic tectum just one week after optic nerve transection. Axotomy induced a prolonged upregulation of FGFR1 and FGFR3 in both retina and tectum. FGFR4 levels decreased in the retina shortly after axotomy, whereas a significant increase was detected in the optic tectum. FGFR2 distribution was not affected by the optic nerve lesion. Changes in the presence of these proteins after axotomy suggest a potential role during regeneration.

Regionally selective activation and differential regulation of ERK, JNK and p38 MAP kinase signalling pathway by protein kinase C in mood modulation

A growing body of evidence indicates that the extracellular signal-regulated kinase (ERK) pathway may participate in the neuronal modulation of depression. p38MAPK and c-Jun-N-terminal kinase/stress-activated protein kinase (JNK/SAPK) also belong to the MAPK family which mainly function as mediators of cellular stresses. Since increasing evidence implicates stress as an important factor in vulnerability to depressive illnesses, the involvement of ERK, JNK and p38MAPK pathways in the modulation of mood was investigated in the forced swim test (FST) and tail suspension test (TST). The effect produced by a single acute session of FST and TST on hippocampal and cortical MAPK expression and phosphorylation was investigated by immunoblotting experiments. In the hippocampus of animals exposed to FST and TST, an intensive, PKC-dependent, ERK1, ERK2, JNK, and p38MAPK phosphorylation was observed. In the frontal cortex, the FST and TST produced a PKC-dependent increase of ERK2 and p38MAPK phosphorylation, a PKC-independent activation of JNK and cAMP response element-binding protein (CREB) whereas any involvement of ERK1 was detected. The PKC blocker calphostin C (0.05-0.1 μg i.c.v.), the MEK inhibitor U0126 (10-20 μg i.c.v.), the p38MAPK inhibitor SB203580 (5-20 μg i.c.v.) and the JNK inhibitor II (0.5-5 μg i.c.v.), produced antidepressant-like behaviour without altering locomotor activity. These results illustrate a differentially mediated activation of MAPK in hippocampus and frontal cortex of animals exposed to behavioural despair paradigms. An antidepressant-like phenotype produced by acute blockade of MAPK signalling was also demonstrated.

PI3K/Akt-sensitive MEK-independent compensatory circuit of ERK activation in ER-positive PI3K-mutant T47D breast cancer cells

We explored the crosstalk between cell survival (phosphatidylinositol 3-kinase (PI3K)/Akt) and mitogenic (Ras/Raf/MEK/extracellular signal-regulated kinase (ERK)) signaling pathways activated by an epidermal growth factor (EGF) and analyzed their sensitivity to small molecule inhibitors in the PI3K-mutant estrogen receptor (ER)-positive MCF7 and T47D breast cancer cells. In contrast to MCF7 cells, ERK phosphorylation in T47D cells displayed resistance to MEK inhibition by several structurally different compounds, such as U0126, PD 098059 and PD 198306, MEK suppression by small interfering RNA (siRNA) and was also less sensitive to PI3K inhibition by wortmannin. Similar effect was observed in PI3K-wild type ER-positive BT-474 cells, albeit to a much lesser extent. MEK-independent ERK activation was induced only by ErbB receptor ligands and was resistant to inhibition of several kinases and phosphatases that are known to participate in the regulation of Ras/mitogen-activated protein kinase (MAPK) cascade. Although single agents against PDK1 or Akt did not affect EGF-induced ERK phosphorylation, a combination of PI3K/Akt and MEK inhibitors synergistically suppressed ERK activation and cellular growth. siRNA-mediated silencing of class I PI3K or Akt1/2 genes also significantly decreased U0126-resistant ERK phosphorylation. Our data suggest that in T47D cells ErbB family ligands induce a dynamic, PI3K/Akt-sensitive and MEK-independent compensatory ERK activation circuit that is absent in MCF7 cells. We discuss candidate proteins that can be involved in this activation circuitry and suggest that PDZ-Binding Kinase/T-LAK Cell-Originated Protein Kinase (PBK/TOPK) may play a role in mediating MEK-independent ERK activation.

Cellular signaling and factors involved in Müller cell gliosis: neuroprotective and detrimental effects

Müller cells are active players in normal retinal function and in virtually all forms of retinal injury and disease. Reactive Müller cells protect the tissue from further damage and preserve tissue function by the release of antioxidants and neurotrophic factors, and may contribute to retinal regeneration by the generation of neural progenitor/stem cells. However, Müller cell gliosis can also contribute to neurodegeneration and impedes regenerative processes in the retinal tissue by the formation of glial scars. This article provides an overview of the neuroprotective and detrimental effects of Müller cell gliosis, with accounts on the cellular signal transduction mechanisms and factors which are implicated in Müller cell-mediated neuroprotection, immunomodulation, regulation of Müller cell proliferation, upregulation of intermediate filaments, glial scar formation, and the generation of neural progenitor/stem cells. A proper understanding of the signaling mechanisms implicated in gliotic alterations of Müller cells is essential for the development of efficient therapeutic strategies that increase the supportive/protective and decrease the destructive roles of gliosis.

Sustained delivery of NT-3 from lens fiber cells in transgenic mice reveals specificity of neuroprotection in retinal degenerations

Several neurotrophic factors (NTFs) are effective in protecting retinal photoreceptor cells from the damaging effects of constant light and slowing the rate of inherited photoreceptor degenerations. It is currently unclear whether, if continuously available, all NTFs can be protective for many or most retinal degenerations (RDs). We used transgenic mice that continuously overexpress the neurotrophin NT-3 from lens fibers under the control of the alphaA-crystallin promoter to test for neuroprotection in light-damage experiments and in four naturally occurring or transgenically induced RDs in mice. Lens-specific expression of NT-3 mRNA was demonstrated both by in situ hybridization in embryos and by reverse-transcriptase polymerase chain reaction (RT-PCR) in adult mice. Furthermore, NT-3 protein was found in abundance in the lens, ocular fluids, and retina by enzyme-linked immunosorbent assay (ELISA) and immunocytochemistry. Overexpression of NT-3 had no adverse effects on the structure or function of the retina for up to at least 14 months of age. Mice expressing the NT-3 transgene were protected from the damaging effects of constant light to a much greater degree than those receiving bolus injections of NT-3. When the NT-3 transgene was transferred into rd/rd, Rds/+, Q344ter mutant rhodopsin or Mertk knockout mice, overexpression of NT-3 had no protective effect on the RDs in these mice. Thus, specificity of the neuroprotective effect of NT-3 is clearly demonstrated, and different molecular mechanisms are inferred to mediate the protective effect in light-induced and inherited RDs.

Effect of high glucose concentration on VEGF and PEDF expression in cultured retinal Müller cells

To explore the effect of high glucose concentration on the expression of vascular endothelial growth factor (VEGF) and pigment epithelium derived factor (PEDF) in the cultured rat retinal Müller cells. Rat Müller cells were cultured and RT-PCR and Western-blot analysis were used to measure the levels of VEGF and PEDF in cultured Müller cells at different high glucose concentrations. Under 10, 20, 30 mmol/L high glucose conditions, the levels of VEGF mRNA and protein increased and the levels of PEDF mRNA and protein decreased. These results suggest that the VEGF and PEDF expression in Müller cells are unbalance under high glucose concentration, which contribute to retinal neovascularization in diabetic retinopathy.

Regulatory mechanisms of fear extinction and depression-like behavior

Human anxiety is frequently accompanied by depression, and when they co-occur both conditions exhibit greater severity and resistance to treatment. Little is known, however, about the molecular processes linking these emotional and mood disorders. Based on previously reported phosphorylation patterns of extracellular signal-regulated kinase (ERK) in the brain, we hypothesized that ERK's upstream activators intertwine fear and mood regulation through their hippocampal actions. We tested this hypothesis by studying the upstream regulation of ERK signaling in behavioral models of fear and depression. Wild-type and ERK1-deficient mice were used to study the dorsohippocampal actions of the putative ERK activators: mitogen-activated and extracellular signal-regulated kinase (MEK), protein kinase C (PKC), and cAMP-dependent protein kinase (PKA). Mice lacking ERK1 exhibited enhanced fear extinction and reduced depression caused by overactivation of ERK2. Both behaviors were reversed by inhibition of MEK, however the extinction phenotype depended on hippocampal, whereas the depression phenotype predominantly involved extrahippocampal MEK. Unexpectedly, inhibition of PKC accelerated extinction and decreased depression by ERK-independent mechanisms, whereas inhibition of PKA did not produce detectable molecular or behavioral effects in the employed paradigm. These results indicate that, contrary to fear conditioning but similar to mood stabilization, extinction of fear required upregulation of MEK/ERK and downregulation of ERK-independent PKC signaling. The dissociation of these pathways may thus represent a common mechanism for fear and mood regulation, and a potential therapeutic option for comorbid anxiety and depression.

Effects of unoprostone on phosphorylated extracellular signal-regulated kinase expression in endothelin-1-induced retinal and optic nerve damage

Endothelin-1 (ET-1), a potent vasoconstrictor peptide, has been implicated in the development of normal- and high-tension glaucoma. We investigated the effects of unoprostone on extracellular signal-regulated kinase (ERK) in ET-1-induced retinal ganglion cell (RGC) death and optic nerve injury. Our morphometric study showed that intravitreal injection of ET-1 led to cell loss in the RGC layer (RGCL) in 28 days. Western blot analysis showed decreased neurofilament (NF) protein in the optic nerve 28 days after ET-1 injection. In this in vivo model, increased phosphorylated ERK (p-ERK) was observed in the retina on 1 day and subsequently in the optic nerve from 7 days after ET-1 injection. Simultaneous injection of M1, as a metabolite of unoprostone, showed further increased p-ERK levels compared with ET-1 injection alone. Our morphometric study of flat-mount preparations stained with cresyl violet or retrograde labeling with a neuro-tracer and Western blot analysis of NF showed that inhibition of ERK phosphorylation led to acceleration of ET-1-induced RGC death and optic nerve damage. In addition, M1 significantly attenuated both RGC loss and the decrease in NF protein induced by ET-1. The protective effects of M1 were significantly inhibited by U0126, an ERK inhibitor. These results suggest that unoprostone has neuroprotective effects against ET-1-induced neuronal injury through ERK phosphorylation.

Progesterone potentiates IP(3)-mediated calcium signaling through Akt/PKB

The activity of cells critically depends on the control of their cytosolic free calcium ion (Ca(2+)) concentration. The objective of the present study was to identify mechanisms of action underlying the control of the gain of intracellular Ca(2+) release by circulating gonadal steroid hormones. Acute stimulation of isolated neurons with progesterone led to IP(3)R-mediated Ca(2+) transients that depend on the activation of the PI3 kinase/Akt/PKB signaling pathway. These results were confirmed at the molecular level and phosphorylation of IP(3)R type 1 by Akt/PKB was identified as the mechanism of action. Hence, it is likely that circulating gonadal steroid hormones control neuronal activity including phosporylation status through receptor- and kinase-mediated signaling. With a direct control of the gain of the Ca(2+) second messenger system as a signaling gatekeeper for neuronal activity the present study identifies a novel pathway for interaction of the endocrine and central nervous system.

Basic fibroblast growth factor-induced protection from light damage in the mouse retina in vivo

Basic fibroblast growth factor (bFGF) has proven neuroprotective efficacy in the rodent retina against a diverse array of injurious stimuli. However, there is no consensus to date as to the molecular mechanisms underlying this neuroprotection. The study presented herein demonstrates increased expression of endogenous bFGF in the albino mouse retina in response to acute exposure to sublethal levels of light stress. The increased expression correlates with significant photoreceptor protection from light damage. The neuroprotection is likely to be mediated by bFGF as we demonstrate that a shorter exposure to bright light stress that does not up-regulate bFGF fails to protect photoreceptors from light damage. Furthermore, intravitreal bFGF injection into the retina of mice 3 h prior to light damage affords almost complete photoreceptor protection from light-induced degeneration. In addition, injected bFGF induces the activation of protein kinase B and extracellular signal-regulated kinase 1/2 signalling which correlate directly with the pathways we find to be activated in response to light stress and up-regulated bFGF. Moreover, we demonstrate that both bright light pre-conditioning and intravitreal bFGF injection result in dramatic increases in levels of inactive glycogen synthase kinase 3beta and cyclic AMP response element binding protein phosphorylation indicating a potential mechanism by which bFGF promotes survival of photoreceptors in vivo.

bFGF promotes photoreceptor cell survival in vitro by PKA-mediated inactivation of glycogen synthase kinase 3beta and CREB-dependent Bcl-2 up-regulation

Although there is substantial evidence supporting the neuroprotective efficacy of basic fibroblast growth factor (bFGF) in the rodent retina there is no consensus to date as to the protective mechanism involved. We hypothesise that bFGF can assert its neuroprotective effects directly on mouse photoreceptors transduced via the activation of specific intracellular signalling pathways. In mouse photoreceptor-derived 661W cells, bFGF promoted a rapid inactivation of glycogen synthase kinase 3beta (GSK3beta) by phosphorylation at Ser9. The effects of bFGF on GSK3beta were dependent on protein kinase A (PKA) activation, as inhibition of this pathway blocked inactivation. Furthermore, bFGF protection against oxidative stress was dependent on PKA inactivation of GSK3beta as PKA inhibition attenuated bFGF-induced protection. Furthermore, transfection of cells with mutant dominant negative GSK3betaS9A that cannot be phosphorylated on Ser9 also abrogated neuroprotection. Activation of the transcription factor cAMP-response element binding protein (CREB) and subsequent up-regulation of Bcl-2 in response to bFGF was also dependent on PKA as inhibition with H-89 attenuated increased pCREB levels and Bcl-2 expression. These results indicate that the protective efficacy of bFGF in mouse photoreceptors involves PKA-dependent inactivation of GSK3beta and subsequent up-regulation of Bcl-2 via CREB activation.

Brain tumor formation in tuberous sclerosis depends on erk activation

Tuberous sclerosis (TS) is an autosomal dominant disease associated with the formation of usually benign tumors or hamartomas. The disease is connected with upregulation of mammalian target of rapamycin, central regulator of protein translation, which is usually regarded to be activated by Akt kinase. Here, we show for the first time that in all four brain lesions and one angiomyolipoma from TS patients both extracellular signal-regulated kinase (Erk) and p90 ribosomal S6 kinase 1 activation as well as Erk-dependent phosphorylation of p70 ribosomal S6 kinase 1 are markedly elevated whereas Akt, participating in the classical pathway of mammalian target of rapamycin activation is not always activated. Erk activation is also present in TS-derived cell lines. Importantly, Erk inhibition leads to the decrease of proliferation potential of such lines. These results show that Erk is specifically implicated in the pathogenesis of hamartomas.

CNTF+BDNF treatment and neuroprotective pathways in the rd1 mouse retina

The rd1 mouse is a relevant model for studying the mechanisms of photoreceptor degeneration in retinitis pigmentosa. Treatment with ciliary neurotrophic factor (CNTF) in combination with brain derived neurotrophic factor (BDNF) is known to rescue photoreceptors in cultured rd1 retinal explants. To shed light on the underlying mechanisms, we studied the effects of 9 days (starting at postnatal day 2) in vitro CNTF+BDNF treatment on the endogenous production of CNTF, BDNF, fibroblast growth factor 2 (FGF2), or the activation of extracellular signal-regulated kinase (ERK), Akt and cAMP-response-element-binding protein (CREB) in retinal explants. In rd1 explants, CNTF+BDNF decreased the number of TUNEL-positive photoreceptors. The treatment also increased endogenous rd1 levels of CNTF and BDNF, but lowered the level of FGF2 expression in rd1 explants. When wild-type explants were treated, endogenous CNTF was similarly increased, while BDNF and FGF2 levels remained unaffected. In addition, treatment of rd1 retinas strongly increased the phosphorylation of ERK, Akt and CREB. In treated wild-type explants, the same parameters were either unchanged (ERK) or decreased (Akt and CREB). The results suggest a role for Akt, ERK and CREB in conveying the neuroprotective effect of CNTF+BDNF treatment in rd1 retinal explants.

Docosahexaenoic acid prevents apoptosis of retina photoreceptors by activating the ERK/MAPK pathway

Identifying the trophic factors for retina photoreceptors and the intracellular pathways activated to promote cell survival is crucial for treating retina neurodegenerative diseases. Docosahexaenoic acid (DHA), the major retinal polyunsaturated fatty acid, prevents photoreceptor apoptosis during early development in vitro, and upon oxidative stress. However, the signaling mechanisms activated by DHA are still unclear. We investigated whether the extracellular signal regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) or the phosphatidylinositol-3-kinase (PI3K) pathway participated in DHA protection. 1,4-Diamino-2,3-dicyano-1,4-bis(2-aminophynyltio) butadiene (U0126), a specific MEK inhibitor, completely blocked the DHA anti-apoptotic effect. DHA rapidly increased ERK phosphorylation in photoreceptors, whereas U0126 blocked this increase. U0126 hindered DHA prevention of mitochondrial depolarization, and blocked the DHA-induced increase in opsin expression. On the contrary, PI3K inhibitors did not diminish the DHA protective effect. DHA promoted the early expression of Bcl-2, decreased Bax expression and reduced caspase-3 activation in photoreceptors. These results suggest that DHA exclusively activates the ERK/MAPK pathway to promote photoreceptor survival during early development in vitro and upon oxidative stress. This leads to the regulation of Bcl-2 and Bax expression, thus preserving mitochondrial membrane potential and inhibiting caspase activation. Hence, DHA, a lipid trophic factor, promotes photoreceptor survival and differentiation by activating the same signaling pathways triggered by peptidic trophic factors.

Phosphoinositide 3-Kinase C2beta regulates cytoskeletal organization and cell migration via Rac-dependent mechanisms

Receptor-linked class I phosphoinositide 3-kinases (PI3Ks) induce assembly of signal transduction complexes through protein-protein and protein-lipid interactions that mediate cell proliferation, survival, and migration. Although class II PI3Ks have the potential to make the same phosphoinositides as class I PI3Ks, their precise cellular role is currently unclear. In this report, we demonstrate that class II phosphoinositide 3-kinase C2beta (PI3KC2beta) associates with the Eps8/Abi1/Sos1 complex and is recruited to the EGF receptor as part of a multiprotein signaling complex also involving Shc and Grb2. Increased expression of PI3KC2beta stimulated Rac activity in A-431 epidermoid carcinoma cells, resulting in enhanced membrane ruffling and migration speed of the cells. Conversely, expression of dominant negative PI3KC2beta reduced Rac activity, membrane ruffling, and cell migration. Moreover, PI3KC2beta-overexpressing cells were protected from anoikis and displayed enhanced proliferation, independently of Rac function. Taken together, these findings suggest that PI3KC2beta regulates the migration and survival of human tumor cells by distinct molecular mechanisms.

GDNF family ligands trigger indirect neuroprotective signaling in retinal glial cells

Apoptotic cell death of photoreceptors is the final event leading to blindness in the heterogeneous group of inherited retinal degenerations. GDNF (glial cell-line-derived neurotrophic factor) was found to rescue photoreceptor function and survival very effectively in an animal model of retinal degeneration (M. Frasson, S. Picaud, T. Leveillard, M. Simonutti, S. Mohand-Said, H. Dreyfus, D. Hicks, and J. Sahel, Investig. Ophthalmol. Vis. Sci. 40:2724-2734, 1999). However, the cellular mechanism of GDNF action remained unresolved. We show here that in porcine retina, GDNF receptors GFRalpha-1 and RET are expressed on retinal Mueller glial cells (RMG) but not on photoreceptors. Additionally, RMG express the receptors for the GDNF family members artemin and neurturin (GFRalpha-2 and GFRalpha-3). We further investigated GDNF-, artemin-, and neurturin-induced signaling in isolated primary RMG and demonstrate three intracellular cascades, which are activated in vitro: MEK/ERK, stress-activated protein kinase (SAPK), and PKB/AKT pathways with different kinetics in dependence on stimulating GFL. We correlate the findings to intact porcine retina, where GDNF induces phosphorylation of ERK in the perinuclear region of RMG located in the inner nuclear layer. GDNF signaling resulted in transcriptional upregulation of FGF-2, which in turn was found to support photoreceptor survival in an in vitro assay. We provide here a detailed model of GDNF-induced signaling in mammalian retina and propose that the GDNF-induced rescue effect on mutated photoreceptors is an indirect effect mediated by retinal Mueller glial cells.

Acute IL-3 priming up-regulates the stimulus-induced Raf-1-Mek-Erk cascade independently of IL-3-induced activation of Erk

IL-3 is a potent priming cytokine for human basophils, inducing an increase of mediator release after stimulation. The mechanism of IL-3 priming of the basophil response to FcepsilonRI aggregating stimuli remains unknown. We explored the regulation of several elements of IgE-mediated signaling by a short priming with IL-3. Early signaling events such as phosphorylation of Syk, Shc, linker for activation of T cells, and the calcium signal were not statistically affected by acute IL-3 priming. Downstream in the signaling cascade, a point of up-regulation was found at the level of Raf-1-Mek-Erk. Although the phosphorylation of Raf-1 was not changed by IL-3 priming, IL-3-primed anti-IgE-stimulated basophils showed a strong synergism for Mek and Erk phosphorylation when compared with either IL-3 or anti-IgE alone; pre-exposure to IL-3 induced a final 13-fold average increase over anti-IgE-induced Erk phosphorylation (6-fold above the sum of anti-IgE and IL-3 alone). The kinetics, dose response, and pharmacologic characteristics of the IL-3 priming of stimulus-induced Erk phosphorylation support the involvement of a yet unknown mechanism that is independent of IL-3-induced Erk and PI3K activation. This type of preactivation can be mimicked by incubation with the Ser-Thr kinase inhibitors, Ro-81-3220, or bisindoylmaleimide II.

Wnt/beta-catenin signaling acts upstream of N-myc, BMP4, and FGF signaling to regulate proximal-distal patterning in the lung

Branching morphogenesis in the lung serves as a model for the complex patterning that is reiterated in multiple organs throughout development. Beta-catenin and Wnt signaling mediate critical functions in cell fate specification and differentiation, but specific functions during branching morphogenesis have remained unclear. Here, we show that Wnt/beta-catenin signaling regulates proximal-distal differentiation of airway epithelium. Inhibition of Wnt/beta-catenin signaling, either by expression of Dkk1 or by tissue-specific deletion of beta-catenin, results in disruption of distal airway development and expansion of proximal airways. Wnt/beta-catenin functions upstream of BMP4, FGF signaling, and N-myc. Moreover, we show that beta-catenin and LEF/TCF activate the promoters of BMP4 and N-myc. Thus, Wnt/beta-catenin signaling is a critical upstream regulator of proximal-distal patterning in the lung, in part, through regulation of N-myc, BMP4, and FGF signaling.

Fibroblast growth factor 2 applied to the optic nerve after axotomy increases Bcl-2 and decreases Bax in ganglion cells by activating the extracellular signal-regulated kinase signaling pathway

We have shown that application of basic fibroblast growth factor (FGF-2) to axotomized optic nerve promotes the survival of frog retinal ganglion cells (RGCs). In the present study we used western blotting and immunocytochemistry to investigate the effects of this FGF-2 treatment upon the activation of the extracellular signal-regulated kinase (ERK) pathway, the amounts and distribution of Bcl-2 family proteins, and the activation of caspase-3. Axotomy alone temporarily increased ERK activation; FGF-2 treatment to the nerve prolonged this activation. This effect was blocked by U0126, a selective ERK kinase (MEK) inhibitor. Axotomy caused a decrease in Bcl-2 and a small increase in Bcl-x(L). FGF-2 treatment caused an ERK-dependent increase in Bcl-2 and an ERK-independent increase in Bcl-x(L). The pro-apoptotic Bax was increased by axotomy; FGF-2 treatment greatly decreased Bax levels, an effect that was inhibited by U0126. Axotomy induced the cleavage of caspase-3; FGF-2 treatment blocked this effect in an ERK-dependent manner. Finally, intraocular application of the MEK inhibitor caused a large reduction in the survival-promoting effect that FGF-2 application to the nerve stump had on RGCs. Our results suggest that FGF-2 acts, at least in part, via the ERK pathway to prevent apoptosis of axotomized RGCs not only by increasing amounts of anti-apoptotic proteins, but also by a striking reduction in the levels of apoptotic effectors themselves.

Contribution of mitogen-activated protein kinases to NMDA-induced neurotoxicity in the rat retina

We examined the contributions of the mitogen-activated protein kinases (MAPKs) family [extracellular signal-regulated kinase (ERK), p38 kinase (p38), and c-Jun N-terminal kinase (JNK)] to N-methyl-D-aspartate (NMDA)-induced neurotoxicity in the rat retina. Detection of apoptotic cell death in the retinal ganglion cell layer (RGCL) and the inner nuclear layer (INL) by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling (TUNEL) staining began 6 h after intravitreal NMDA (100 nmol) injection and continued to increase thereafter. Western blot analysis showed that phosphorylated MAPKs (p-MAPKs) were expressed in the retina following a temporal manner: maximal expression of phosphorylated ERK (p-ERK) at 1 h, maximal expression of phosphorylated p38 (p-p38) at 6 h, and beginning of phosphorylated JNK (p-JNK) significant increase at 6 h after injection. An immunohistochemical/TUNEL co-localization study showed that p-JNK- and p-p38-positive cells in the RGCL were frequently TUNEL-positive, whereas few p-ERK-positive cells were TUNEL-positive. Moreover, co-injection of inhibitors for JNK (0.2 nmol SP600125) and/or p38 (2.0 nmol SB203580) with NMDA was effective in ameliorating NMDA-induced apoptotic cell loss in the RGCL 12 h after injection, as shown by TUNEL-positive cell counts. These inhibitors also protected the inner retina as shown by morphometric studies such as cell counts in the RGCL and measurement of the IPL thickness 7 days after injection. On the other hand, an ERK inhibitor (2.0 nmol U0126) did not suppress NMDA-induced cell death in the RGCL nor thinning of the IPL. These findings suggest that JNK and p38 are proapoptotic in NMDA-induced cell death in the RGCL, but not ERK.

Extracellular signal-regulated kinase 1/2 mediates survival, but not axon regeneration, of adult injured central nervous system neurons in vivo

Neurotrophins play important roles in the response of adult neurons to injury. The intracellular signaling mechanisms used by neurotrophins to regulate survival and axon growth in the mature CNS in vivo are not well understood. The goal of this study was to define the role of the extracellular signal-regulated kinases 1/2 (Erk1/2) pathway in the survival and axon regeneration of adult rat retinal ganglion cells (RGCs), a prototypical central neuron population. We used recombinant adeno-associated virus (AAV) to selectively transduce RGCs with genes encoding constitutively active or wild-type mitogen-activated protein kinase kinase 1 (MEK1), the upstream activator of Erk1/2. In combination with anterograde and retrograde tracing techniques, we monitored neuronal survival and axon regeneration in vivo. MEK1 gene delivery led to robust and selective transgene expression in multiple RGC compartments including cell bodies, dendrites, axons and targets in the brain. Furthermore, MEK1 activation induced in vivo phosphorylation of Erk1/2 in RGC bodies and axons. Quantitative analysis of cell survival demonstrated that Erk1/2 activation promoted robust RGC neuroprotection after optic nerve injury. In contrast, stimulation of the Erk1/2 pathway was not sufficient to induce RGC axon growth beyond the lesion site. We conclude that the Erk1/2 pathway plays a key role in the survival of axotomized mammalian RGCs in vivo, and that activation of other signaling components is required for axon regeneration in the growth inhibitory CNS environment.

ERK2 is required for FGF1-induced JNK1 phosphorylation in Xenopus oocyte expressing FGF receptor 1

A possible connection between the ERK2 and JNK1 MAP kinases transduction cascades was investigated in Xenopus oocytes expressing FGFR1 stimulated by FGF1. Injection of various inhibitors for the Shc/Grb2/Ras/Mos/MEK/ERK2 cascade blocked FGF1-induced germinal vesicle breakdown (GVBD), as well as ERK2 and JNK1 phosphorylation. JNK1 was found to be activated downstream of ERK2, since injection of an active ERK2 triggered JNK1 phosphorylation and inhibition of ERK2 either by a MEK inhibitor or the MKP3 phosphatase blocked JNK1 phosphorylation. These results demonstrated that in FGFR1 signalling JNK1 phosphorylation depends on ERK2.

Regulation of extracellular signal-regulated kinase by homocysteine in hippocampus

In several neurological disorders including hyperhomocysteinemia, homocysteine (Hcy) accumulates in the brain, and acts as a potent neurotoxin. However, the molecular mechanisms induced by increased levels of Hcy in brain are not well understood. Here we show an activation of the extracellular signal-regulated kinases (ERK1 and ERK2) and the downstream nuclear targets Elk-1 and calcium/cAMP response element binding protein, in the hippocampus of cystathionine beta synthase deficient mice, a murine model of hyperhomocysteinemia. An ex vivo model of hippocampal slices allowed us to reproduce Hcy -induced ERK activation and to unravel the mechanisms responsible of this activation. Of interest, N-methyl-d-aspartate (NMDA), non-NMDA and metabotropic glutamate receptor antagonists all blocked Hcy -induced ERK activation. Moreover, the ERK activation was blocked in the presence of Na+-channel blocker tetrodotoxin, indicating the existence of a trans-synaptic activity in ERK activation by Hcy in hippocampal slices. The effects of Hcy on ERK cascade activation were also dependent on calcium influx, CaMK-II, PKC as well as PKA activation. Thus, altogether these data support a role of Hcy on ERK activation, via complex mechanisms, starting with a control of glutamate release, which in turn activates ionotropic and metabotropic receptor subtypes and produces increases in intracellular calcium levels.

Neural regeneration in the chick retina

In warm-blooded vertebrates, possibilities for retinal regeneration have recently become reality with the discovery of neural stem cells in the mature eye. A number of different cellular sources of neural stem cells have been identified. These sources include stem cells at the retinal margin, pigmented cells in the ciliary body and iris, non-pigmented cells in the ciliary body and Müller glia within the retina. This review focuses on recent reports of neural stem cells and regeneration in the postnatal chicken retina. In the chicken eye sources of neurogensis and regeneration include: (1) retinal stem cells at the peripheral edge of the retina; (2) Müller glia in central regions of the retina; (3) non-pigmented epithelial cells in the posterior portion of the ciliary body; and (4) possibly pigmented cells in the pars plana of the ciliary body. This review discusses the similarities between the retinal progenitor cells in the postnatal eye and those found in the embryo. In addition, I discuss combinations of growth factors, (insulin, IGF-I, EGF and FGF2) that are capable of stimulating the proliferation and production of neurons from neural progenitors, non-neural epithelial cells, and postmitotic support cells in the avian eye. In summary, the mechanisms that regulate the proliferation and differentiation of cells with neurogenic potential are beginning to be understood and the postnatal chicken eye has proven to be a useful model system to study retinal regeneration.

The minimal transactivation domain of the basic motif-leucine zipper transcription factor NRL interacts with TATA-binding protein

The basic motif-leucine zipper (bZIP) transcription factor NRL controls the expression of rhodopsin and other phototransduction genes and is a key mediator of photoreceptor differentiation. To delineate the molecular mechanisms underlying transcriptional initiation of rod-specific genes, we characterized different regions of the NRL protein using yeast-based autoactivation assays. We identified 35 amino acid residues in the proline- and serine-rich N-terminal region (called minimal transactivation domain, MTD), which, when combined with LexA or Gal4 DNA binding domains, exhibited activation of target promoters. Because this domain is conserved in all proteins of the large Maf family, we hypothesized that NRL-MTD played an important role in assembling the transcription initiation complex. Our studies showed that the NRL protein, including the MTD, interacted with full-length or the C-terminal domain of TATA-binding protein (TBP) in vitro. NRL and TBP could be co-immunoprecipitated from bovine retinal nuclear extract. TBP was also part of c-Maf and MafA (two other large Maf proteins)-containing complex(es) in vivo. Our data suggest that the function of NRL-MTD is to activate transcription by recruiting or stabilizing TBP (and consequently other components of the general transcription complex) at the promoter of target genes, and a similar function may be attributed to other bZIP proteins of the large Maf family.

Gap junctional communication is required to maintain mouse cortical neural progenitor cells in a proliferative state

The mechanisms that determine whether neural stem cells remain in a proliferative state or differentiate into neurons or glia are largely unknown. Here we establish a pivotal role for gap junction-mediated intercellular communication in determining the proliferation and survival of mouse neural progenitor cells (NPCs). When cultured in the presence of basic fibroblast growth factor (bFGF), NPCs express the gap junction protein connexin 43 and are dye-coupled. Upon withdrawal of bFGF, levels of connexin 43 and dye coupling decrease, and the cells cease proliferating and differentiate into neurons; the induction of gap junctions by bFGF is mediated by p42/p44 mitogen-activated protein kinases. Inhibition of gap junctions abolishes the ability of bFGF to maintain NPCs in a proliferative state resulting in cell differentiation or cell death, while overexpression of connexin 43 promotes NPC self-renewal in the absence of bFGF. In addition to promoting their proliferation, gap junctions are required for the survival of NPCs. Gap junctional communication is therefore both necessary and sufficient to maintain NPCs in a self-renewing state.

Characterization of the basic fibroblast growth factor-evoked proliferation of the human Müller cell line, MIO-M1

BACKGROUND

Basic fibroblast growth factor (bFGF) has been suggested to mediate activation of Müller glial cells in the ischemic-hypoxic retina. However, the intracellular pathways activated by bFGF in human Müller cells have been little explored. We characterized the signaling transduction pathways which are involved in the control and growth factor-evoked proliferation of a recently described human Müller cell line, MIO-M1. In addition, we investigated whether bFGF evoked the release of vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF) from the cells.

METHODS

The growth factor-evoked proliferation of cultured MIO-M1 cells was estimated by means of a bromodeoxyuridine immunoassay, in the absence and presence of blockers of mitogen-activated protein kinases (MAPKs) and of the phosphatidylinositol-3 kinase (PI3K). The activation state of the p44/p42 MAPK was determined by Western blotting, and the bFGF-evoked release of VEGF and HGF was evaluated by ELISA.

RESULTS

bFGF evoked a concentration-dependent increase of the cell proliferation, with an EC50 of approximately 1 ng/ml, via activation of both the p44/p42 MAPK and the p38 MAPK. In contrast, the mitogenic effects of the platelet-derived and the heparin-binding epidermal growth factors were dependent on p44/p42 MAPK activation and independent of activation of p38 MAPK. The transforming growth factors beta1 and beta2 also evoked cell proliferation which was independent of activation of the MAPKs investigated. bFGF evoked a release of VEGF and of HGF by the cells; these effects were independent of MAPK activation and were possibly mediated by activation of the PI3K signaling pathway.

CONCLUSION

bFGF evokes multiple intracellular signaling pathways in human Müller cells which underlie the gliotic cell responses upon ischemic-hypoxic insults in the retina. Beside the stimulation of cell proliferation, which is dependent on activation of p44/p42 and p38 MAPKs, bFGF induces the secretion of VEGF and HGF by Müller cells.

Mitogen-activated protein kinase signaling in the hippocampus and its modulation by corticotropin-releasing factor receptor 2: a possible link between stress and fear memory

A coordinated activation of multiple interlinked signaling pathways involving cAMP-dependent protein kinase (PKA) and mitogen-activated extracellular signal-regulated kinases (Mek-1/2) regulates gene expression and neuronal changes underlying memory consolidation. In the present study we investigated whether these molecular cascades might mediate the effects of stress on memory formation. We also investigated the role of hippocampal corticotropin-releasing factor receptor 2 (CRF2) in stress-enhanced learning and molecular signaling mediated by PKA, Mek-1/2, and their downstream targets extracellularly regulated kinases 1 and 2 (Erk-1/2) and p90-ribosomal-s-kinase-1 (p90Rsk-1). Acute 1 hr immobilization was used as a stressful stimulus, and one-trial context-dependent fear conditioning was used as a model for associative learning. Training of BALB/c mice 3 hr after the end of immobilization resulted in an enhancement of conditioned fear, as indicated by significantly increased freezing behavior of stressed when compared with nonstressed mice. Interestingly, Erk-1/2 phosphorylation after conditioning of nonstressed and stressed mice depended on PKA and Mek-1/2, respectively. Intrahippocampal injection of the selective Mek-1/2 inhibitor U0126 or CRF2 antagonist antisauvagine-30 (aSvg-30) prevented stress-enhanced fear conditioning and Mek-1/2-dependent activation of Erk-1/2 and p90Rsk-1. aSvg-30 did not affect the phosphorylation of the PKA regulatory subunit II of stressed mice. The molecular and behavioral effects of CRF2 coincided with stress-induced upregulation of CRF2 mRNA. These results suggest that modulation of Mek-1/2-dependent signaling by hippocampal CRF2 can be selectively involved in the delayed effects of stress on memory consolidation.

Neuronal MEK is important for normal fear conditioning in mice

The extracellular signal-regulated kinase (ERK) cascade has received much attention for its possible role in neuronal synaptic plasticity. Although ERK activation has been linked to learning behaviors and activity-dependent neuronal function, much of the acquired data has relied upon pharmacological agents that suppress ERK function in both neurons and nonneuronal cells. To determine the function of neuronal ERK activity in learning, a new line of transgenic mice was generated wherein dominant-negative MEK1, the upstream obligate activator of ERK1/2, was expressed by using a neuronal-specific and pan-neuronal Talpha1 alpha-tubulin promoter element. Mice expressing this construct exhibited decreased ERK1/2 activity in the hippocampus and thus were tested for learning impairments. In a battery of control tests, including open field, rotarod, and shock threshold, the transgenic mice displayed no deficits and performed as well as their wild-type littermate counterparts. However, the mice displayed a significant impairment in contextual fear conditioning compared with the wild-type littermates. These findings indicate that the MEK1/ERK1/2 cascade within neurons plays an important role in the processes of learning and memory.

Retinal neuroprotection by growth factors: a mechanistic perspective

For more than a decade it has been known that certain growth factors inhibit apoptosis in genetically determined and experimental models of inner and outer retinal degeneration. The molecular mechanisms underlying these protective effects and the signaling that supports the survival of photoreceptors and retinal ganglion cells in these models have recently come under more in depth investigation. This paper reviews our current understanding of the balance of pro- and antiapoptotic signals that determine cell fate in the retina and how the activation of key signal transduction pathways by specific classes of neurotrophins protects retinal neurons.

The fibroblast growth factor receptor is at the site of convergence between mu-opioid receptor and growth factor signaling pathways in rat C6 glioma cells

Mitogenic signaling of G protein-coupled receptors (GPCRs) can proceed via sequential epidermal growth factor receptor (EGFR) transactivation and extracellular signal-regulated kinase (ERK) phosphorylation. Although the mu-opioid receptor (MOR) mediates stimulation of ERK via EGFR transactivation in human embryonic kidney 293 cells, the mechanism of acute MOR signaling to ERK has not been characterized in rat C6 glioma cells that seem to contain little EGFR. Herein, we describe experiments that implicate fibroblast growth factor (FGF) receptor (FGFR) transactivation in the convergence of MOR and growth factor signaling pathways in C6 cells. MOR agonists, endomorphin-1 and morphine, induced a rapid (3-min) increase of ERK phosphorylation that was abolished by MOR antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2. By using selective inhibitors and overexpression of dominant negative mutants, data were obtained to suggest that MOR signaling to ERK is transduced by Gbetagamma and entails Ca2+- and protein kinase C-mediated steps, whereas the FGFR branch of the pathway is Ras-dependent. An intermediary role of FGFR1 transactivation was suggested by MOR- but not kappa-opioid receptor (KOR)-induced FGFR1 tyrosine phosphorylation. A dominant negative mutant of FGFR1 attenuated MOR- but not KOR-induced ERK phosphorylation. Thus, a novel transactivation mechanism entailing secreted endogenous FGF may link the GPCR and growth factor pathways involved in MOR activation of ERK in C6 cells.

Exogenous growth factors stimulate the regeneration of ganglion cells in the chicken retina

Recent reports have found that the posthatch chicken retina has the capacity for neuronal regeneration. The purpose of this study was to test whether the types of cells destroyed by neurotoxic lesions influence the types of cells that are regenerated, and whether exogenous growth factors stimulate neural regeneration in the chicken retina. N-methyl-D-aspartate (NMDA) was used to destroy amacrine and bipolar cells; kainate was used to destroy bipolar, amacrine, and ganglion cells; colchicine was used to selectively destroy ganglion cells. Following toxin-induced damage, bromo-deoxyuridine was used to label proliferating cells. In some animals, growth factors were injected into the vitreous chamber of the eye. We found that the proliferation of cells within the retina was stimulated by toxin-induced cell loss, and by insulin and FGF2. After either kainate- or colchicine-induced retinal damage, some of the newly generated cells expressed markers and had the morphology of ganglion cells. The combination of insulin and FGF2 stimulated the regeneration of ganglion cells in kainate- and colchicine-treated retinas. We conclude that exogenous growth factors can be used to stimulate neural regeneration in the retina. We propose that the type of neuron destroyed in the retina may allow or promote the regeneration of that neuronal type.

Role of phosphatidylinositide 3-kinase in brain-derived neurotrophic factor-induced DARPP-32 expression in medium size spiny neurons in vitro

Brain-derived neurotrophic factor (BDNF) regulates several properties of striatal dopaminoceptive medium-sized spiny neurons (MSNs) in vivo and in vitro, including expression levels of DARPP-32 (dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein, 32 kDa). DARPP-32 is expressed in 96% of the MSNs, and is a key modulator of dopamine actions. We investigated the intracellular signal transduction pathways activated by BDNF in MSNs and via which BDNF induces DARPP-32 expression. We found that phosphorylation of the cyclic AMP response element binding protein (CREB) is only transiently increased following stimulation of MSNs by BDNF, whereas increased phosphorylation of the extracellular signal regulated kinases 1 and 2 (Erk1/2) and Akt is sustained for longer than 4 h. Treatment of cultures with inhibitors of mitogen-activated protein kinase kinase (MEK) or phosphatidylinositide 3-kinase (PI3K) showed that the majority of the BDNF-induced increase in DARPP-32 requires the PI3K pathway. We also found that inhibition of PI3K reduces BDNF-induced Erk phosphorylation, indicating that cross-talk between these pathways may play a prominent role in MSNs.