Signal-regulated proteins and fibroblast growth factor receptors: comparative immunolocalization in rat retina

Ccrk-Mak/Ick signaling is a ciliary transport regulator essential for retinal photoreceptor survival

Primary cilia are microtubule-based sensory organelles whose dysfunction causes ciliopathies in humans. The formation, function, and maintenance of primary cilia depend crucially on intraflagellar transport (IFT); however, the regulatory mechanisms of IFT at ciliary tips are poorly understood. Here, we identified that the ciliopathy kinase Mak is a ciliary tip-localized IFT regulator that cooperatively acts with the ciliopathy kinase Ick, an IFT regulator. Simultaneous disruption of Mak and Ick resulted in loss of photoreceptor ciliary axonemes and severe retinal degeneration. Gene delivery of Ick and pharmacological inhibition of FGF receptors, Ick negative regulators, ameliorated retinal degeneration in Mak -/- mice. We also identified that Ccrk kinase is an upstream activator of Mak and Ick in retinal photoreceptor cells. Furthermore, the overexpression of Mak, Ick, and Ccrk and pharmacological inhibition of FGF receptors suppressed ciliopathy-related phenotypes caused by cytoplasmic dynein inhibition in cultured cells. Collectively, our results show that the Ccrk-Mak/Ick axis is an IFT regulator essential for retinal photoreceptor maintenance and present activation of Ick as a potential therapeutic approach for retinitis pigmentosa caused by MAK mutations.

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.

The increase in retinal cells proliferation induced by FGF2 is mediated by tyrosine and PI3 kinases

Since 1973, multiple effects of basic fibroblast growth factor have been described in a large number of cells. These effects include proliferation, survival and differentiation. The aim of this work was to study the intracellular pathways involved in the basic fibroblast growth factor (FGF2) effect on rat retinal cells proliferation in vitro. Our data show that treatment with FGF2 increases proliferation in a concentration- and time-dependent manner. The effect of 25 ng/ml FGF2 was blocked by 10 microM genistein, a tyrosine kinase inhibitor and by 25 microM LY294002, a PI3 kinase inhibitor. The concomitant treatment with 0.3 microM chelerythrine chloride, a protein kinase C inhibitor, and 6.25 microM LY294002 also inhibited the effect of FGF2. Our results suggest that the proliferative effect of FGF2 on retinal cell cultures involves the activation of distinct kinases.

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.

Adeno-associated viruses containing bFGF or BDNF are neuroprotective against excitotoxicity

PURPOSE

Brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (bFGF) hold much promise for the protection of retinal ganglion cells against excitotoxic cell death. We tested the possibility of delivering these growth factors to retinal ganglion cells via an adeno-associated viral (AAV) vector and tested their efficacy in two models of excitotoxicity.

METHODS

Rat retinas were infected with AAV vectors encoding bFGF or BDNF. A control vector containing green fluorescent protein (GFP) was injected in the contralateral eye. Eyes were subjected to either an intravitreal injection of N-methyl-D-aspartate (NMDA) or optic nerve crush, and ganglion cell survival was evaluated.

RESULTS

AAV.CMV.bFGF and AAV.CBA.BDNF were neuroprotective against NMDA injection 1 month post-treatment. Additionally, AAV.CMV.bFGF was protective against optic nerve crush.

CONCLUSION

AAV-mediated delivery of bFGF and BDNF can promote retinal cell survival following excitotoxic insult.

Fibroblast growth factor-2 gene delivery stimulates axon growth by adult retinal ganglion cells after acute optic nerve injury

Basic fibroblast growth factor (or FGF-2) has been shown to be a potent stimulator of retinal ganglion cell (RGC) axonal growth during development. Here we investigated if FGF-2 upregulation in adult RGCs promoted axon regrowth in vivo after acute optic nerve injury. Recombinant adeno-associated virus (AAV) was used to deliver the FGF-2 gene to adult RGCs providing a sustained source of this neurotrophic factor. FGF-2 gene transfer led to a 10-fold increase in the number of axons that extended past 0.5 mm from the lesion site compared to control nerves. Detection of AAV-mediated FGF-2 protein in injured RGC axons correlated with growth into the distal optic nerve. The response to FGF-2 upregulation was supported by our finding that FGF receptor-1 (FGFR-1) and heparan sulfate (HS), known to be essential for FGF-2 signaling, were expressed by adult rat RGCs. FGF-2 transgene expression led to only transient protection of injured RGCs. Thus the effect of this neurotrophic factor on axon extension could not be solely attributed to an increase in neuronal survival. Our data indicate that selective upregulation of FGF-2 in adult RGCs stimulates axon regrowth within the optic nerve, an environment that is highly inhibitory for regeneration. These results support the hypothesis that key factors involved in axon outgrowth during neural development may promote regeneration of adult injured neurons.

Developmental plasticity of photoreceptors

During development, retinal ganglion cells undergo conspicuous structural remodeling as they gradually attain their mature morphology and connectivity. Alterations in their dendritic organization and in their axonal projections can also be achieved following early insult to their targets or their afferents. Other retinal cell types are thought not to display this same degree of developmental plasticity. The present review will consider the evidence, drawn largely from recent experimental studies in the carnivore retina, that photoreceptors also undergo structural remodeling, extending their terminals transiently into inner plexiform layer before retracting to the outer plexiform layer. The determinants of this transient targeting to the inner plexiform layer are considered, and the role of cholinergic amacrine cells is discussed. The factors triggering this retraction are also considered, including the concurrent maturational changes in outer segment formation and in the differentiation of the outer plexiform layer. These results provide new insight into the life history of the photoreceptor cell and its connectivity, and suggest a transient role for the photoreceptors in the circuitry of the inner retina during early development, prior to the onset of phototransduction.

Possible involvement of a fibroblast growth factor 9 (FGF9)-FGF receptor-3-mediated pathway in adult pig retinal ganglion cell survival in vitro

The expression and potential roles of fibroblast growth factors (FGF) and their cognate FGF receptors (FGFR) in adult mammalian retinal ganglion cells (RGC) are poorly known. We show that FGFR-3 and FGFR-4 are especially pronounced on RGC and amacrine cell bodies in adult pig inner retinae both in vivo and in vitro. Western blotting revealed distinct profiles for each receptor. Expression of each FGFR and effects of the preferred ligand for FGFR-3, FGF9, upon RGC survival and neurite outgrowth were examined in primary retinal cell cultures: whereas there was no stimulation of neuritogenesis, RGC survival was promoted in a dose-dependent manner (ED(50) approximately 500 pg/ml, mean maximal increase of 60%) and could be completely blocked by addition of FGF9 neutralising antibody. Experiments with three additional FGF (FGF1, FGF2, and FGF4) showed no stimulation of RGC survival above control levels. Taken together, these data suggest that the ligand-receptor couple FGF9-FGFR-3 may function to promote survival of adult mammalian RGC, and their application might be beneficial in retinal degenerative diseases such as glaucoma.

Two animal models of retinal degeneration are rescued by recombinant adeno-associated virus-mediated production of FGF-5 and FGF-18

The goal of these experiments was to evaluate the potential of the fibroblast growth factor family members FGF-5 and FGF-18 to rescue photoreceptors from cell death in retinal degenerative disease. Two strains of transgenic rats, expressing either a P23H or an S334ter rhodopsin mutation, were used as model systems. The neurotrophic growth factors were delivered by subretinal injection of adeno-associated virus vectors, driving expression of the genes with a constitutive CMV promoter. Morphological and functional analyses were performed to determine whether FGF-5 or FGF-18 overexpression could ameliorate cell death in the retina. Immunocytochemistry was used to determine the cellular sites of expression of the factors and to test for up-regulation of FGF receptors due to injection. Significant rescue from photoreceptor cell death was found after injections of vectors expressing either FGF-5 or FGF-18 in the animal models. Increased survival of photoreceptors did not produce a significant increase in electroretinographic responses, however, reflecting either trauma due to the surgery or a suppression of signaling due to expression of proteins. Three weeks after injections, both growth factors were localized to the inner and outer segments of photoreceptors, and the receptors FGFR1 and FGFR2 were also found to be up-regulated in these regions. No visible pathological changes were seen in the FGF-5- or FGF-18-treated eyes. These results indicate that the delivery of either FGF-5 or FGF-18 with adeno-associated virus protects photoreceptors from apoptosis in transgenic rat models of retinitis pigmentosa and that the rescue is probably mediated by conventional receptor tyrosine kinase pathways in photoreceptors.

Rod outer segment maintenance is enhanced in the presence of bFGF, CNTF and GDNF

We employed a morphological assay of outer segment collapse to determine if growth factors or other supplements directly affect dissociated rod photoreceptors in vitro. The morphological changes in outer segments were correlated with the light responsiveness of rods. Time-lapse video microscopy was used to observe the collapse of rod outer segments from isolated single cells and small clumps of cells. A consistent pattern of outer segment collapse into the inner segment was observed, yielding a convenient assay of the effects of neurotrophic factors on photoreceptor functional maintenance. The functional state of rods, defined as light-responsiveness, was measured with suction electrode recordings and matched with the various stages of outer segment collapse. Ciliary neurotrophic factor (CNTF) and glial cell-line-derived neurotrophic factor (GDNF) at a high concentration, yielded statistically significant improvements in rat outer segment survival times. Basic fibroblast growth factor (bFGF), which rescues photoreceptors in several rodent models of retinal degeneration, produced a significant increase in survival time in the presence of the cofactor heparin. In 4 out of 10 cases using human tisue, bFGF also yielded a significant increase in survival times. When brain-derived neurotrophic factor (BDNF) was applied to rat rods, outer segment survival times did not change. Outer segments collapsed more quickly when either pigment epithelial cell derived factor (PEDF) or sugar N-acetyl D-galactosamine (NAD-gal) were present. Our results show that rod photoreceptors can respond to bFGF, GDNF and CNTF in vitro and provide evidence for a direct effect of these neurotrophic factors on rods. The rapid collapse of isolated photoreceptors in this model provides a convenient means for testing various neurotrophic agents and the induced cellular responses.