Growth factors in combination, but not individually, rescue rd mouse photoreceptors in organ culture

Interactions of cyclic adenosine monophosphate, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor treatment on the survival and growth of postnatal mesencephalic dopamine neurons in vitro

The survival of rat postnatal mesencephalic dopamine (DA) neurons in dissociated cell cultures was studied by examining the combinatorial effects of dibutyryl cyclic adenosine monophosphate (db-cAMP), glial cell line-derived neurotrophic factor (GDNF), and brain-derived neurotrophic factor (BDNF), as well as selective inhibitors of protein kinase A (PKA), and mitogen-activated protein kinase (MAPK). Postnatal DA neurons were maintained for 14 days in vitro, and were identified by immunohistochemistry using tyrosine hydroxylase antibody. The survival and growth of DA neurons was significantly increased by the inclusion of either >100 microM db-cAMP or 10 microM Forskolin plus 100 microM IBMX in the culture medium. Neither 10-50 ng/ml GDNF nor 50 ng/ml BDNF alone significantly increased DA neuron survival in vitro. However, the combined use of GDNF and BDNF did increase DA neuron survival, and the addition of either db-cAMP or IBMX/Forskolin to media containing these neurotrophins markedly increased DA neuron survival and growth. The cAMP inhibitor Rp-cAMP, the cAMP-dependent protein kinase A inhibitor H89, and the MAP kinase (MAPK) pathway inhibitor PD98059 significantly reduced the survival of DA neurons when applied alone in the absence of added growth factors. Application of GDNF plus BDNF, or db-cAMP significantly protected the DA neurons from the deleterious effects on survival of either 20 microM H89 or 20 microM PD 98059. The results suggest that BDNF, GDNF, and cAMP produce convergent signals to activate PKA and MAPK pathways which are involved in the survival of postnatal mesencephalic DA neurons in vitro.

Involvement of two different cell death pathways in retinal atrophy of cathepsin D-deficient mice

To understand the mechanisms of retinal atrophy in cathepsin D-deficient mice, the postnatal development of their retinae was analyzed. TUNEL-positive cells appeared abundantly in the outer nuclear layer (ONL) and slightly in the inner nuclear layer (INL). Nitric oxide synthase (NOS) was induced in microglial cells which invaded retinal layers and phagocytosed dead cell debris, while NOS inhibitors prevented cell death in the INL but not in the ONL. Caspases 9 and 3 were activated only in the ONL after P15. Moreover, no atrophic change was detected in the retina of mice deficient in cathepsin B or L. These results suggest that cathepsin D is essential for the metabolic maintenance of retinal photoreceptor cells and that its deficiency induces apoptosis of the cells, while the loss of INL neurons is mediated by NO from microglial cells.

TGFbeta induces GDNF responsiveness in neurons by recruitment of GFRalpha1 to the plasma membrane

We have previously shown that the neurotrophic effect of glial cell line-derived neurotrophic factor (GDNF) in vitro and in vivo requires the presence of transforming growth factor (TGF)beta. Using primary neurons (chick E8 ciliary) we show that the combination of GDNF plus TGFbeta promotes survival, whereas the single factors do not. This cooperative effect is inhibited by blocking the extracellular signal-regulated kinase (ERK)/MAPK pathway, but not by interfering with the PI3 kinase signaling cascade. Although there is no functional GDNF signaling in the absence of TGFbeta, pretreatment with TGFbeta confers GDNF responsiveness to the cells. This is not due to upregulation of GDNF receptors mRNA and protein, but to TGFbeta-induced recruitment of the glycosyl-phosphatidylinositol-anchored GDNF receptor (GFR)alpha1 to the plasma membrane. This is supported by the fact that GDNF in the presence of a soluble GFRalpha1 can promote survival in the absence of TGFbeta. Our data suggest that TGFbeta is involved in GFRalpha1 membrane translocation, thereby permitting GDNF signaling and neurotrophic effects.

Morphological and functional abnormalities in the inner retina of the rd/rd mouse

We investigated the effects of photoreceptor degeneration on the anatomy and physiology of inner retinal neurons in a mouse model of retinitis pigmentosa, the retinal degeneration (rd) mutant mouse. Although there is a general assumption that the inner retinal cells do not suffer from photoreceptor death, we confirmed major changes both accompanying and after this process. Changes include sprouting of horizontal cells, lack of development of dendrites of rod bipolar cells, and progressive atrophy of dendrites in cone bipolar cells. Electrophysiological recordings demonstrate a selective impairment of second-order neurons that is not predictable on the basis of a pure photoreceptor dysfunction. Our data point out the necessity to prove integrity of the inner retina before attempting restoring visual function through photoreceptor intervention. This is even more important when considering that although intervention can be performed before the onset of any symptoms in animals carrying inherited retinopathies, this is obviously not true for human subjects.

Neurturin enhances the survival of axotomized retinal ganglion cells in vivo: combined effects with glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor

In the present study we localized glial cell line-derived neurotrophic factor (GDNF), and the high affinity receptor for GDNF (GFRalpha-1) in the rat retina. We also examined the effects of neurturin on the survival of axotomized retinal ganglion cells (RGCs) and compared neurturin-mediated RGC rescue to GDNF and brain-derived neurotrophic factor (BDNF) neuroprotection. We administered combined injections of neurturin with BDNF or GDNF in order to determine if these factors rescue RGCs by different mechanisms. GDNF immunoreactivity was localized to RGCs, photoreceptors, and retinal pigment epithelial cells. GFRalpha-1 immunoreactivity was localized to RGCs, Müller cells, and photoreceptors. RGC densities in control retinas decreased from the original value of 2481+/-121 (RGCs/mm(2)+/-S.D.) to 347+/-100 at 14 days post-axotomy. Neurturin treatment significantly increased RGC survival after axotomy (745+/-94) similar to GDNF (868+/-110). BDNF treatment resulted in higher RGC survival (1109+/-156) than either neurturin or GDNF. Combined administration of neurturin with BDNF had additive effects on the survival of axotomized RGCs (1962+/-282), similar to combined administration of GDNF and BDNF (1825+/-269). Combined administration of neurturin and GDNF (1265+/-178) had an enhanced effect on RGC survival. These results suggest that neurturin, GDNF, and BDNF act independently to rescue injured RGCs. Our results also suggest that RGCs and retinal Müller cells may be responsive to GDNF because they both express GFRalpha-1. The present findings have implications for the rescue of injured retinal ganglion cells, as well as other CNS neurons that are responsive to neurturin, GDNF, and BDNF, including midbrain dopaminergic neurons and motor neurons.

Dopamine has a critical role in photoreceptor degeneration in the rd mouse

Photoreceptors receive paracrine input from dopaminergic interplexiform cells. Rod photoreceptors in the rd mouse degenerate rapidly due to a specific gene defect. We investigated the effects of dopamine on rd mouse photoreceptors in retinal organ culture. Retinas were harvested from rd or wild-type mice at postnatal day 2 and grown in organ culture for 27 days. When antagonists for either D(1)- or D(2)-family dopamine receptors were added to the media, photoreceptor degeneration was blocked. Furthermore, when dopamine was depleted by the addition of 6-hydroxydopamine and pargyline, photoreceptor survival appeared comparable to wild-type retinal cultures. The addition of a dopamine agonist induced photoreceptor degeneration in dopamine-depleted rd organ cultures. In all cases, photoreceptors maintained robust staining of opsin. These results demonstrate that dopamine antagonists or dopamine depletion blocks photoreceptor degeneration and that dopamine is necessary for photoreceptor degeneration in the rd mouse retinal organ culture model, indicating that dopamine antagonists may represent a therapeutic strategy in retinal degenerative disease.

Astrocyte delivery of glial cell line-derived neurotrophic factor in a mouse model of Parkinson's disease

Primary astrocytes were genetically modified ex vivo to express recombinant glial cell line-derived neurotrophic factor (GDNF) and subsequently were tested for their ability to provide neuroprotection to dopaminergic neurons in a 6-hydroxydopamine (6-OHDA) mouse model of Parkinson's disease. A replication-defective retrovirus was constructed, which contained the rat GDNF sequence and a sequence encoding a beta-galactosidase (beta-gal)/neomycin phosphotransferase fusion protein, linked via an internal ribosomal entry site. Murine astrocytes transduced with this vector secreted GDNF into the culture media at the rate of 115 +/- 34 pg/24 h/10(5) cells and expressed cytoplasmic beta-gal, whereas control nontransduced astrocytes were negative for GDNF production and cytoplasmic beta-gal expression. Mice that received implants of GDNF-producing astrocytes into the striatum or nigra displayed elevated levels of GDNF compared to mice that received control nontransduced astrocytes. In addition, tissue content of GDNF was increased bilaterally and in brain regions both proximal and distal to the graft, even though astrocyte migration away from the graft site did not occur. Importantly, GDNF-producing astrocytes provided marked neuroprotection of nigral dopaminergic perikarya, and partial protection of striatal dopaminergic fibers, when implanted into the midbrain 6 days prior to a retrograde 6-OHDA lesion, as assessed by tyrosine hydroxylase immunohistochemistry. Similarly, GDNF-producing astrocytes prevented the acquisition of amphetamine-induced rotational behavior in 6-OHDA-treated mice and completely prevented dopamine depletion within the substantia nigra, as assessed by high-performance liquid chromatography. These results indicate that continuous exposure to low levels of GDNF provided by transgenic astrocytes provides marked neuroprotection of nigral dopaminergic neurons. (c)2002 Elsevier Science (USA).

A sensory neuron subpopulation with unique sequential survival dependence on nerve growth factor and basic fibroblast growth factor during development

We characterized a subpopulation of dorsal root ganglion (DRG) sensory neurons that were previously identified as preferential targets of enkephalins. This group, termed P-neurons after their "pear" shape, sequentially required nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) for survival in vitro during different developmental stages. Embryonic P-neurons required NGF, but not bFGF. NGF continued to promote their survival, although less potently, up to postnatal day 2 (P2). Conversely, at P5, they needed bFGF but not NGF, with either factor having similar effects at P2. This trophic switch was unique to that DRG neuronal group. In addition, neither neurotrophin-3 (NT-3) nor brain-derived neurotrophic factor influenced their survival during embryonic and postnatal stages, respectively. The expression of NGF (Trk-A) and bFGF (flg) receptors paralleled the switch in trophic requirement. No single P-neuron appeared to coexpress both Trk-A and flg. In contrast, all of them coexpressed flg and substance P, providing a specific marker of these cells. Immunosuppression of bFGF in newborn animals greatly reduced their number, suggesting that the factor was required in vivo. bFGF was present in the DRG and spinal cord, as well as in skeletal muscle, the peripheral projection site of P-neurons, as revealed by tracer DiIC(18)3. The lack of requirement of NT-3 for survival and immunoreactivity for the neurofilament of 200 kDa distinguished them from muscle proprioceptors, suggesting that they are likely to be unmyelinated muscle fibers. Collectively, their properties indicate that P-neurons constitute a distinct subpopulation of sensory neurons for which the function may be modulated by enkephalins.

Long-term protection of retinal structure but not function using RAAV.CNTF in animal models of retinitis pigmentosa

The present study aimed to determine whether intravitreal administration of an adeno-associated virus (AAV) carrying ciliary neurotrophic factor (CNTF) can achieve long-term morphological and physiological rescue of photoreceptors in animal models of retinitis pigmentosa, and whether injection of this virus after degeneration begins is effective in protecting the remaining photoreceptors. We injected rAAV.CNTF.GFP intravitreally in early postnatal Prph2(Rd2/Rd2) (formerly rds/rds) mice and in adult P23H and S334ter rhodopsin transgenic rats. Contralateral eyes received an intravitreal injection of rAAV.GFP or a sham injection. We evaluated the eyes at 6 months (rats) and 8.5 to 9 months (mice) postinfection and looked for histological and electoretinographic (ERG) evidence of photoreceptor rescue and CNTF-GFP expression. Intravitreal administration of rAAV resulted in efficient transduction of retinal ganglion cells in the Prph2(Rd2/Rd2) retina, and ganglion, Muller, and horizontal/amacrine cells in the mutant rat retinas. Transgene expression localized to the retinal region closest to the injection site. We observed prominent morphological protection of photoreceptors in the eyes of all animals receiving rAAV.CNTF.GFP. We found the greatest protection in regions most distant from the CNTF-GFP-expressing cells. The Prph2(Rd2/Rd2) ERGs did not exhibit interocular differences. Eyes of the rat models administered rAAV.CNTF.GFP had lower ERG amplitudes than those receiving rAAV.GFP. The discordance of functional and structural results, especially in the rat models, points to the need for a greater understanding of the mechanism of action of CNTF before human application can be considered.

Mouse retina explants after long-term culture in serum free medium

The neonatal mouse retina remains viable as an explant in serum-supplemented growth media for more than 4 weeks. Interpretation of drug effects on this tissue is compromised by the enigmatic composition of the serum. We sought to remove this ambiguity by culturing neonatal as well as late postnatal mouse retina in serum-free nutrient medium. In this study three important observations were made, (1) there is histotypic development of neonatal as well as preservation of late postnatal mouse retinal structure during long-term culture in serum-free medium, although the late postnatal tissue tends to show some loss of cells in the outer nuclear layer. (2) Protein expression in explant photoreceptor cells was similar to that in the litter-matched ones, except for green cone opsin and interphotoreceptor retinoid-binding protein, although mRNA of the latter is present at similar amounts as in age-matched in vivo controls. (3) Cells of the inner retina stained by antibodies to calcium-binding proteins display some novel sprouting of processes. The results show that the mouse retina can be cultured as an explant for more than 4 weeks in a serum-free medium. This represents an important step forward because, (1) the possibility of interference of drug effects by unknown serum factors has been eliminated; and (2) the spent culture medium can be analyzed to investigate biomolecules released by the retina in vitro.

Lens epithelium-derived growth factor (LEDGF) delays photoreceptor degeneration in explants of rd/rd mouse retina

Lens epithelium derived growth factor (LEDGF) has been shown to rescue embryonic chick photoreceptor cells from serum starvation and heat stress, light damaged photoreceptor cells in Lewis rats, and photoreceptor cells in RCS rats. The aim of our study is to study the rescue effect of LEDGF on photoreceptor cells in the rd/rd mouse using our long-term serum free organ culture. At the end of this culture period of 21-26 days LEDGF treated rd mouse retina showed an increased photoreceptor survival compared to the untreated controls. LEDGF has no effect on expression and localization of opsin and arrestin in the rod photoreceptor cells when RPE is present. The protective potency of LEDGF on the retinal photoreceptor cells is similar to that of BDNF. LEDGF is known to activate heat shock proteins (Hsps) and the elevated Hsps are also reported to suppress apoptosis.

Effects of docosahexaenoic acid on retinal development: cellular and molecular aspects

We have recently shown that docosahexaenoic acid (DHA) is necessary for survival and differentiation of rat retinal photoreceptors during development in vitro. In cultures lacking DHA, retinal neurons developed normally for 4 d; then photoreceptors selectively started an apoptotic pathway leading to extensive degeneration of these cells by day 11. DHA protected photoreceptors by delaying the onset of apoptosis; in addition, it advanced photoreceptor differentiation, promoting opsin expression and inducing apical differentiation in these neurons. DHA was the only fatty acid having these effects. Mitochondrial damage accompanied photoreceptor apoptosis and was markedly reduced upon DHA supplementation. This suggests that a possible mechanism of DHA-mediated photoreceptor protection might be the preservation of mitochondrial activity; a critical amount of DHA in mitochondrial phospholipids might be required for proper functioning of these organelles, which in turn might be essential to avoid cell death. Müller cells in culture appeared to be involved in DHA processing: they took up DHA, incorporated it into glial phospholipids, and channeled it to photoreceptors in coculture. Both Müller cells, when cocultured with neuronal cells, and the glial-derived neurotrophic factor (GDNF) protected photoreceptors from cell death. These results suggest that glial cells may play a central role in regulating photoreceptor survival during development through the provision of trophic factors. The multiple effects of DHA on photoreceptors suggest that, in addition to its structural role, DHA might be one of the trophic factors required by these cells.

Cell transplantation as a treatment for retinal disease

It has been shown that photoreceptor degeneration can be limited in experimental animals by transplantation of fresh RPE to the subretinal space. There is also evidence that retinal cell transplants can be used to reconstruct retinal circuitry in dystrophic animals. Here we describe and review recent developments that highlight the necessary steps that should be taken prior to embarking on clinical trials in humans.

The regrowth of axons within tissue defects in the CNS is promoted by implanted hydrogel matrices that contain BDNF and CNTF producing fibroblasts

In this study we demonstrate the potential for combining biocompatible polymers with genetically engineered cells to elicit axon regrowth across tissue defects in the injured CNS. Eighteen- to 21-day-old rats received implants of poly N-(2-hydroxypropyl)-methacrylamide (HPMA) hydrogels containing RGD peptide sequences that had been infiltrated with control (untransfected) fibroblasts (n = 8), fibroblasts engineered to express brain-derived neurotrophic factor (BDNF) (n = 5), ciliary neurotrophic factor (CNTF) (n = 5), or a mixture of BDNF and CNTF expressing fibroblasts (n = 11). Fibroblasts were prelabeled with Hoechst 33342. Cell/polymer constructs were inserted into cavities made in the left optic tract, between thalamus and superior colliculus. After 4-8 weeks, retinal projections were analyzed by injecting right eyes with cholera toxin (B-subunit). Rats were perfused 24 h later and sections were immunoreacted to visualize retinal axons, other axons (RT97 antibody), host astrocytes and macrophages, donor fibroblasts, and extracellular matrix molecules. The volume fraction (VF) of each gel that was occupied by RT97(+) axons was quantified. RT-PCR confirmed expression of the transgenes prior to, and 5 weeks after, transplantation. Compared to control rats (mean VF = 0.02 +/- 0.01% SEM) there was increased ingrowth of RT97(+) axons into implants in CNTF (mean VF = 0.33 +/- 0.19%) and BDNF (mean VF = 0.62 +/-0.19%) groups. Axon growth into hydrogels in the mixed BDNF/CNTF group (mean VF = 3.58 +/- 0.92%) was significantly greater (P < 0.05) than in the BDNF or CNTF fibroblast groups. Retinal axons exhibited a complex branching pattern within gels containing BDNF or BDNF/CNTF fibroblasts; however, they regrew the greatest distances within implants containing both BDNF and CNTF expressing cells.

AAV-mediated delivery of ciliary neurotrophic factor prolongs photoreceptor survival in the rhodopsin knockout mouse

Retinitis pigmentosa (RP), an inherited retinal degenerative disease causing blindness, is characterized by progressive apoptotic death of photoreceptors. Therapeutic modification of photoreceptor apoptosis may provide an effective therapy for this disorder. Ciliary neurotrophic factor (CNTF) has been shown to promote survival of a number of different neuronal cell types, including photoreceptors. The present study aimed to test whether adeno-associated virus (AAV)-mediated delivery of the gene encoding CNTF delays photoreceptor death in the rhodopsin knockout (opsin(-/-)) mouse, an animal model of RP. The vector was made to express a secretable form of CNTF in tandem with a marker GFP. Cultured 293 cells transduced with this virus expressed both CNTF and GFP. The conditioned media from such cells supported the survival of chick dorsal root ganglion neurons in the same manner as recombinant CNTF. Subretinal administration of this virus led to efficient transduction of photoreceptors as indicated by GFP fluorescence and CNTF immunostaining. Histologic examination showed significant photoreceptor preservation in the injected quadrant of the retina. This protection lasted through termination of the experiment (3 months). AAV-mediated delivery of CNTF may have implications for the treatment of human retinal degeneration.

Deafferentation-induced apoptosis of neurons in thalamic somatosensory nuclei of the newborn rat: critical period and rescue from cell death by peripherally applied neurotrophins

This study shows that unilateral transection of the infraorbital nerve (ION) in newborn (P0) rats induces apoptosis in the contralateral ventrobasal thalamic (VB) complex, as evidenced by terminal transferase-mediated deoxyuridine triphosphate-biotin nick end labelling (TUNEL) and electron miscroscopy. Double-labelling experiments using retrograde transport of labelled microspheres injected into the barrel cortex, followed by TUNEL staining, show that TUNEL-positive cells are thalamocortical neurons. The number of TUNEL-positive cells had begun to increase by 24 h postlesion, increased further 48 h after nerve section, and decreased to control levels after 120 h. Lesion-induced apoptosis in the VB complex is less pronounced if ION section is performed at P4, and disappears if the lesion is performed at P7. This time course closely matches the critical period of lesion-induced plasticity in the barrel cortex. Nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF), applied on the ION stump alone or in combination, are able to partially rescue thalamic neurons from apoptosis. Total cell counts in the VB complex of P7 animals that underwent ION section at P0 confirm the rescuing effect of BDNF and NGF. Blockade of axonal transport in the ION mimics the effect of ION section. These data suggest that survival-promoting signals from the periphery, maybe neurotrophins, are required for the survival of higher-order neurons in the somatosensory system during the period of fine-tuning of neuronal connections. We also propose that anterograde transneuronal degeneration in the neonatal rat trigeminal system may represent a new animal model for studying the pathways of programmed cell death in vivo.