Glial cell line-derived neurotrophic factor rescues target-deprived sympathetic spinal cord neurons but requires transforming growth factor-beta as cofactor in vivo

Fibroblast growth factor-2 requires glial-cell-line-derived neurotrophic factor for exerting its neuroprotective actions on glutamate-lesioned hippocampal neurons

FGF-2 is a potent neurotrophic factor for several populations of CNS neurons and has been shown to protect hippocampal neurons from glutamate-induced cell death in vitro and in vivo. Mechanisms underlying the neurotrophic and protective actions of FGF-2 have been resolved only in part. Using glutamate-treated cultured hippocampal neurons we show that FGF-2 shares its neuroprotective capacity with GDNF. Hippocampal neurons express glial-cell-line-derived neurotrophic factor (GDNF), its receptors c-Ret and the lipid-anchored GDNF family receptor-alpha1 (GFRalpha-1), and the FGF receptor 1 (FGFR I). Neutralizing antibodies to GDNF abolish the neuroprotective effect of FGF-2. In support of the notion that GDNF is required to permit the protective effects of FGF-2 we find that FGF-2 up-regulates GDNF and GFRalpha-1 in hippocampal neurons. Furthermore, FGF-2-induced GDNF causes enhanced phosphorylation of c-Ret and the signaling components Akt and Erk. A putative downstream target of FGF-2 and GDNF are bcl-2 gene family members, whose mRNAs are differentially up-regulated by the two factors. Together, these data suggest that GDNF is an important protective factor for glutamate-lesioned hippocampal neurons and an essential mediator of the neuroprotective actions of FGF-2.

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).

Chromaffin cell mitogenesis by neurturin and glial cell line-derived neurotrophic factor

Neurturin and glial cell line-derived neurotrophic factor are novel mitogens for normal adult rat chromaffin cells in vitro. These neurotrophic factors differ from the previously described adult chromaffin cell mitogens, nerve growth factor and basic fibroblast growth factor, in that their effects are potentiated by depolarization and activation of protein kinase C. Neurturin and glial cell line-derived neurotrophic factor signal via the receptor tyrosine kinase, ret, but may also act independently of ret. Both depolarization and phorbol esters act synergistically with neurturin to up-regulate ret protein expression in chromaffin cell cultures, suggesting a mechanism for potentiation of mitogenesis. However, a direct role for ret in mitogenesis has not been established. Stimulation by neurturin causes increased phosphorylation of extracellular signal-regulated kinases 1 and 2 in cultured chromaffin cells, and mitogenesis is prevented by inhibitors of their phosphorylation. Inhibitors of phosphatidylinositol 3-kinase also prevent mitogenesis. The present findings suggest the hypothesis that neurotrophic factors and neurally derived signals might cooperatively regulate chromaffin cell proliferation in vivo in the rat. In addition, trans-synaptic stimulation might provide a route by which epigenetic factors could influence the development of adrenal medullary hyperplasia in humans with hereditary multiple endocrine neoplasia syndromes 2A and 2B by affecting expression and/or activation of ret.

Transforming growth factor-beta and forskolin attenuate the adverse effects of long-term Schwann cell denervation on peripheral nerve regeneration in vivo

Transforming growth factor-beta (TGF-beta) plays a central role in the regulation of Schwann cell (SC) proliferation and differentiation and is essential for the neurotrophic effects of several neurotrophic factors (reviewed by Unsicker and Krieglstein, 2000; Unsicker and Strelau, 2000). However, its role in peripheral nerve regeneration in vivo is not yet understood. Our studies were carried out to characterize (1) the effects of duration of regeneration, and chronic SC denervation on the number of tibial (TIB) motor neurons that regenerated axons over a fixed distance (25 mm into distal common peroneal [CP] nerve stumps), and (2) the effect of in vitro incubation of 6-month chronically denervated sciatic nerve explants with TGF-beta and forskolin on their capacity to support axonal regeneration in vivo. TIB--CP cross-suture in Silastic tubing was used, and regeneration into 0-24-week chronically denervated CP stumps was allowed for either 1.5 or 3 months. Chronically denervated rat sciatic nerve explants (3 x 3 mm(2)) were incubated in vitro with either DMEM and 15% fetal calf serum (D-15) plus TGF-beta/forskolin or D-15 alone for 48 h and placed into a 10-mm Silastic tube that bridged the proximal and distal nerve stumps of a freshly cut TIB nerve. The number of tibial motor neurons that regenerated axons through the explants and 25 mm into the distal nerve stump after 6 months, and TIB regeneration into the CP nerve stumps, were assessed using retrograde tracers, fluorogold, or fluororuby. We found that all tibial motor neurons regenerate their axons 25 mm into 0-4-week denervated CP nerve stumps after a regeneration period of 3 months. Reducing regeneration time to 1.5 months and chronic denervation, reduced the number of motor neurons that regenerated axons over 25 mm. Exposure of 6-month denervated nerve explants to TGF-beta/forskolin increased the number of motor neurons that regenerated through them from 258 +/-13; mean +/- SE to 442 +/- 22. Hence, acute treatment of atrophic SC with TGF-beta can reactivate the growth-permissive SC phenotype to support axonal regeneration.

GDNF and NGF released by synthetic guidance channels support sciatic nerve regeneration across a long gap

The present work was performed to determine the ability of neurotrophic factors to allow axonal regeneration across a 15-mm-long gap in the rat sciatic nerve. Synthetic nerve guidance channels slowly releasing NGF and GDNF were fabricated and sutured to the cut ends of the nerve to bridge the gap. After 7 weeks, nerve cables had formed in nine out of ten channels in both the NGF and GDNF groups, while no neuronal cables were present in the control group. The average number of myelinated axons at the midpoint of the regenerated nerves was significantly greater in the presence of GDNF than NGF (4942 +/-1627 vs. 1199 +/-431, P < or = 0.04). A significantly greater number of neuronal cells in the GDNF group, when compared to the NGF group, retrogradely transported FluoroGold injected distal to the injury site before explantation. The total number of labelled motoneurons observed in the ventral horn of the spinal cord was 98.1 +/-23.4 vs. 20.0 +/-8.5 (P < or = 0.001) in the presence of GDNF and NGF, respectively. In the dorsal root ganglia, 22.7% +/- 4.9% vs. 3.2% +/-1.9% (P +/-0.005) of sensory neurons were labelled retrogradely in the GDNF and NGF treatment groups, respectively. The present study demonstrates that, sustained delivery of GDNF and NGF to the injury site, by synthetic nerve guidance channels, allows regeneration of both sensory and motor axons over long gaps; GDNF leads to better overall regeneration in the sciatic nerve.

TGF-beta and the regulation of neuron survival and death

Transforming growth factor-betas (TGF-betas) constitute a superfamily of multifunctional cytokines with important implications in morphogenesis, cell differentiation, and tissue remodeling. In the developing nervous system, TGF-beta2 and -beta3 occur in radial and astroglial cells as well as in many populations of postmitotic, differentiating neurons. TGF-beta1 is restricted to the choroid plexus and meninges. In addition to functions related to glial cell maturation and performances, TGF-beta2 and -beta3 are important regulators of neuron survival. In contrast to neurotrophic factors, as for example, neurotrophins, TGF-betas are most likely not neurotrophic by themselves. However, they can dramatically increase the potency of select neurotrophins, fibroblast growth factor-2, ciliary neurotrophic factor, and glial cell line-derived neurotrophic factor (GDNF). In the case of GDNF, we have shown that GDNF fails to promote the survival of highly purified neuron populations in vitro unless it is supplemented with TGF-beta. This also applies to the in vivo situation, where antibodies to all three TGF-beta isoforms fully prevent the trophic effect of GDNF on axotomized, target-deprived neurons. In addition to the TGF-beta isoforms -beta2 and -beta3, other members of the TGF-beta superfamily are expressed in the nervous system having important roles in embryonic patterning, cell migration, and neuronal transmitter determination. We have cloned and expressed a novel TGF-beta, named growth/differentiation factor-15 (GDF-15). GDF-15 is synthesized in the choroid plexus and released into the CSF, but also occurs in all regions investigated of the developing and adult brain. GDF-15 is a potent trophic factor for developing and 6-OHDA-lesioned midbrain dopaminergic neurons in vitro and in vivo, matching the potency of GDNF.

Functional regeneration in a rat Parkinson's model after intrastriatal grafts of glial cell line-derived neurotrophic factor and transforming growth factor beta1-expressing extra-adrenal chromaffin cells of the Zuckerkandl's organ

Intrabrain transplantation of chromaffin cell aggregates of the Zuckerkandl's organ, an extra-adrenal paraganglion that has never been tested for antiparkinsonian treatment, induced gradual improvement of functional deficits in parkinsonian rats. These beneficial effects were related to long survival of grafted cells, striatal reinnervation, and enhancement of dopamine levels in grafted striatum. Grafted cells were not dopaminergics, but they expressed glial cell line-derived neurotrophic factor (GDNF) and transforming growth factor-beta(1). These factors were detected in the host striatal tissue, indicating that chromaffin cells secreted them after grafting. Because glial cell line-derived neurotrophic factor possesses neurorestorative properties over dopaminergic neurons, and transforming growth factor-beta(1) is a cofactor that potentiates the neurotrophic actions of GDNF, functional regeneration was likely caused by the chronic trophic action of neurotrophic factors delivered by long-surviving grafted cells. This work should stimulate research on the clinical applicability of transplants of the Zuckerkandl's organ in Parkinson's disease.

Transforming growth factor-beta1 enhances expression of brain-derived neurotrophic factor and its receptor, TrkB, in neurons cultured from rat cerebral cortex

The effects of transforming growth factor (TGF)-beta1 on expression of brain-derived neurotrophic factor (BDNF) and its high-affinity receptor, TrkB, in neurons cultured from the cerebral cortex of 18-day-old embryonic rats were examined. BDNF mRNA was significantly increased from 24-48 hr after the TGF-beta1 treatment over 20 ng/ml. Accumulation of BDNF protein in the culture medium was also potentiated by TGF-beta1, although the intracellular content of BDNF was nearly unchanged. The enhancement of BDNF mRNA expression was suppressed by the co-presence of decorin, a small TGF-beta-binding proteoglycan that inhibits the biological activities of TGF-betas. mRNA expression of full-length TrkB, the bioactive high-affinity receptor for BDNF, was also upregulated after treatment with TGF-beta1. These observations suggest that: 1) TGF-beta1 potentiates BDNF/TrkB autocrine or local paracrine system; and 2) the neurotrophic activity of TGF-beta1 is partly responsible for the BDNF induced by TGF-beta1 itself. To test this latter possibility, we examined the neuronal survival activity of TGF-beta1 with or without K252a, a selective inhibitor of Trk family tyrosine kinases. TGF-beta1 significantly enhanced neuronal survival, but the co-presence of K252a completely suppressed the activity, demonstrating the involvement of Trk receptor signaling in TGF-beta1-mediated neuronal survival in cultured rat cortical neurons. These results seem to be in line with recent findings by other investigators that some neurotrophic factors including BDNF require TGF-betas as a cofactor to exert their neurotrophic activities.

Growth and neurotrophic factors regulating development and maintenance of sympathetic preganglionic neurons

The functional anatomy of sympathetic preganglionic neurons is described at molecular, cellular, and system levels. Preganglionic sympathetic neurons located in the intermediolateral column of the spinal cord connect the central nervous system with peripheral sympathetic ganglia and chromaffin cells inside and outside the adrenal gland. Current knowledge is reviewed of the development of these neurons, which share their origin with progenitor cells, giving rise to somatic motoneurons in the ventral horn. Their connectivities, transmitters involved, and growth factor receptors are described. Finally, we review the distribution and functions of trophic molecules that may have relevance for development and maintenance of preganglionic sympathetic neurons.

Distribution of TGF-beta, the TGF-beta type I receptor and the R-II receptor in peripheral nerves and mechanoreceptors; observations on changes after traumatic injury

The mechanisms governing the regeneration of denervated peripheral mechanoreceptors are similar to those of peripheral nerves. The ability to regenerate depends partly on changes of the Schwann cell phenotype. The transforming growth factor beta (TGF-beta) family have been implicated in induction of Schwann cell proliferation, production of extracellular matrix and neurotrophin synthesis as well as synthesis or repression of cell adhesion molecules. Hence, they may prove to be of importance for regenerative mechanisms in peripheral mechanoreceptors. The distribution of TGF-beta, the receptors I and II and intra-cellular second messengers, Smad 2/3 and 4 was assessed in sensory neurones, peripheral nerves and mechanoreceptors by immuno-histochemistry, immuno-electron microscopy and in situ hybridisation. TGF-beta2 mRNA and TGF-beta2-like immunoreactivity (IR) were expressed in injured small and medium sized rat sensory neurones of dorsal root ganglia. TGF-beta and receptor II mRNA and immunoreactivities (IR) were present in satellite cells. Intact and injured sensory neurones expressed receptor I mRNA and Smad 2 mRNA. TGF-beta2 mRNA was found in transected nerve stumps and in sensory mechanoreceptors. TGF-beta1, 2 and Smad 4 were also observed in inner core lamellar cells of intact and denervated cat Pacinian corpuscles. Lamellar cells of intact and denervated Meissner corpuscles were TGF-beta immunoreactive. Merkel cells were receptors I and II immunoreactive. In conclusion, cutaneous and subcutaneous mechanoreceptors differ with regard to the expression of TGF-beta isoforms and receptors.

Trophic dependencies of rodent corticospinal neurons

According to the classical neurotrophin hypothesis, neuronal survival is regulated by limited access to target-derived neurotrophic substances. Recent studies have indicated that this regulation is more complex than originally thought. First, neurons are not only supported by target-derived molecules but also via anterograde, paracrine, and autocrine mechanisms. Second, phenotypes of neurotrophic factor-/receptor-mutant animals displayed fewer neuronal deficits than predicted, suggesting interactivity and redundancy of trophic support of neurons. Finally, certain neurotrophins, in addition to their survival-promoting action, are able to induce neuronal death. Observations in the corticospinal system support the general applicability of these concepts and provide additional insights into the integrative mode of neuronal survival regulation. CNTF and GDNF support developing corticospinal neurons (CSN) by direct mechanisms, while the effects of NT-4/5 require cell contacts of CSN with other cortical neurons in vitro. Thus, these effects do not merely reflect trophic redundancy but the ability of CSN to integrate survival signals of growth factors from different families via different pathways. CNTF and GDNF also promote survival of adult axotomized CSN in vivo. Virtually all adult CSN express mRNA coding for the NT-3-receptor TrkC and the BDNF-receptor TrkB, and after axotomy, CSN also express mRNA for the common neurotrophin-receptor p75NTR, suggesting a role of endogenous neurotrophins for survival regulation of CSN. Indeed, most axotomized CSN depend on endogenous BDNF for survival, and endogenous NT-3 promotes the death of BDNF-dependent CSN. NT-3-mediated death-induction requires co-signalling of TrkC- and p75NTR-receptors. With BDNF/TrkB promoting survival and NT-3/TrkC/p75NTR promoting death, CSN integrate at least three different neurotrophin/receptor-signals for death/survival decisions.

TGF-β modulates programmed cell death in the retina of the developing chick embryo

Programmed cell death (PCD) is a key phenomenon in the regulation of cell number in multicellular organisms. We have shown that reduction of endogenous transforming growth factor β (TGF-β) prevents apoptotic PCD of neurons in the developing peripheral and central nervous system, suggesting that TGF-β is an important mediator of ontogenetic neuron death. Previous studies suggested that there are other pro-apoptotic molecules, nerve growth factor (NGF) and brain-derived neurotrophic factor, that induce cell death in the nervous system. In the developing chick retina, NGF induces PCD by activation of the p75 receptor. We have studied the role of TGF-β and its putative interdependence with NGF-mediated PCD in the chick retina. We found that TGF-β is present in the developing chick retina during the period of PCD and is essentially required to regulate PCD of retinal cells. TGF-β2, TGF-β3 and the ligand-binding TGF-β receptor can be detected immunocytochemically in the central retina, a region where apoptosis is most prominent during the early period of PCD. Application of a TGF-β-neutralizing antibody to chick embryos in ovo resulted in a decrease in the number of TUNEL-positive cells and a reduction of free nucleosome levels. In terms of magnitude, reduction of PCD caused by the neutralization of endogenous TGF-β was equivalent to that seen after anti-NGF application. Neutralization of both factors did not result in a further decrease in apoptosis, indicating that NGF and TGF-β may act on the same cell population. Furthermore, neutralization of TGF-β did not affect the expression of NGF or the p75-receptor. Our results suggest that TGF-β and NGF are both required to regulate cell death in the chick retina in vivo.

Glial cell line-derived neurotrophic factor (GDNF) and its receptor complex are expressed in the auditory nerve of the mature rat cochlea

Glial cell line-derived neurotrophic factor (GDNF) is a survival factor for many neuronal cell types which signals through a heterodimer receptor consisting of GDNF-family receptor alpha 1 (GFRalpha-1) and Ret (rearranged during transformation). GDNF expression has previously been reported in the inner hair cells of the rat cochlea, with expression of GFRalpha-1 but not Ret in the cell bodies of the auditory nerve (spiral ganglion cells), using in situ hybridization. The present study used reverse transcription-polymerase chain reaction (RT-PCR), and immunocytochemistry to examine GDNF, GFRalpha-1 and Ret in the adult rat auditory nerve. Semi-quantitative RT-PCR showed expression of GDNF and the two receptor components, GFRalpha-1 and Ret, in the modiolar subfraction of the cochlea containing spiral ganglion cells. A shorter mRNA splice variant for GDNF was also detected. Immunocytochemistry showed immunostaining in the modiolus for GDNF, GFRalpha-1 and Ret that was confined to spiral ganglion cells. When RT-PCR expression levels were compared to the expression in the substantia nigra, GFRalpha-1 expression levels were similar, Ret mRNA was lower in the modiolus and GDNF expression was higher in the modiolus. However, when GDNF was further assessed using Western blot, while GDNF protein was found in the modiolus it was at lower levels than in substantia nigra tissue. These results demonstrate that GDNF and both of its receptor components are found in spiral ganglion cells of the adult rat cochlea. Along with the previous report of GDNF in inner hair cells, these new results provide a basis for the role of GDNF as a survival factor for the auditory nerve, as suggested by previous studies.

Lack of neurotrophin-4 causes selective structural and chemical deficits in sympathetic ganglia and their preganglionic innervation

Neurotrophin-4 (NT-4) is perhaps the still most enigmatic member of the neurotrophin family. We show here that NT-4 is expressed in neurons of paravertebral and prevertebral sympathetic ganglia, i.e., the superior cervical (SCG), stellate (SG), and celiac (CG) ganglion. Mice deficient for NT-4 showed a significant reduction (20-30%) of preganglionic sympathetic neurons in the intermediolateral column (IML) of the thoracic spinal cord. In contrast, neuron numbers in the SCG, SG, and CG were unchanged. Numbers of axons in the thoracic sympathetic trunk (TST) connecting the SG with lower paravertebral ganglia were also reduced, whereas axon numbers in the cervical sympathetic trunk (CST) were unaltered. Axon losses in the TST were paralleled by losses of synaptic terminals on SG neurons visualized by electron microscopy. Furthermore, immunoreactivity for the synaptic vesicle antigen SV2 was clearly reduced in the SG and CG. Levels of catecholamines and tyrosine hydroxylase immunoreactivity were dramatically reduced in the SG and the CG but not in the SCG. Despite this severe phenotype in the sympathetic system, blood pressure levels were not reduced and displayed a pattern more typical of deficits in baroreceptor afferents. Numbers of IML neurons were unaltered at postnatal day 4, suggesting a postnatal requirement for their maintenance. In light of these and previous data, we hypothesize that NT-4 provided by postganglionic sympathetic neurons is required for establishing and/or maintaining synapses of IML neurons on postganglionic cells. Impairment of synaptic connectivity may consequently reduce impulse flow, causing a reduction in transmitter synthesis in postganglionic neurons.

Brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor are required simultaneously for survival of dopaminergic primary sensory neurons in vivo

Null mutations affecting members of the transforming growth factor-beta and neurotrophin families result in overlapping patterns of neuronal cell death. This is particularly striking in the cranial sensory nodose-petrosal ganglion complex (NPG), in which loss of either glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or neurotrophin-4 (NT-4) results in a 30-50% reduction in neuronal survival. It is unknown, however, whether GDNF and any single neurotrophin support survival of the same cells, and if so, whether they are required simultaneously or sequentially during development. To approach these issues we defined survival requirements of nodose and petrosal neurons for GDNF in vitro and in bdnf, gdnf, and bdnf/gdnf null mutant mice, as well as the distribution of GDNF in NPG target tissues. Our analyses focused on the total population of ganglion cells as well as the subset of NPG neurons that are dopaminergic. Neuron losses in bdnf/gdnf double mutants are not additive of the losses in single bdnf or gdnf null mutants, indicating that many cells, including dopaminergic neurons, require both GDNF and BDNF for survival in vivo. Moreover, both factors are required during the same period of development, between embryonic day (E) 15.5 and E17.5. In addition, GDNF, like BDNF is expressed in target tissues at the time of initial target innervation and coincident with GDNF dependence of the innervating neurons. Together, these findings demonstrate that both GDNF and BDNF can act as target-derived trophic factors and are required simultaneously for survival of some primary sensory neurons.

Autonomic neurogenesis and apoptosis are alternative fates of progenitor cell communities induced by TGFbeta

The question of how appropriate cell types are generated in correct numbers during development of the peripheral nervous system has become particularly intriguing with the identification of multipotent progenitor cells in postmigratory targets of the neural crest. Recently, we have provided evidence that community effects in response to factors of the TGFbeta family might represent a mechanism to suppress inappropriate nonneural fates from multipotent progenitors in developing peripheral ganglia. In culture, BMP2 and TGFbeta promote neurogenesis at the expense of a smooth-muscle-like fate in clusters of neural-crest-derived multipotent progenitor cells. We now show that the neurons generated by TGFbeta factors belong to the autonomic lineage and that cells within the developing sympathetic ganglia express TGFbeta-type II receptor. In addition to its neurogenic activity, TGFbeta but not BMP2 also induces apoptosis as an alternative fate in cultured progenitor communities. Interestingly, these fate decisions are controlled by graded changes in TGFbeta concentrations: lower doses of TGFbeta promote neurogenesis while slightly higher doses induce predominantly apoptosis. These effects of TGFbeta are specific for an early developmental stage since progenitor cells lose their competence to respond to the proapoptotic activity of TGFbeta upon neuronal differentiation. In vivo, the expression of TGFbeta3 in differentiated neurons suggests that the signal concentration gradually increases with the number of neurons formed in the autonomic ganglia. We propose that TGFbeta functions in a biphasic manner during autonomic gangliogenesis to control both neurogenesis and subsequently the number of neurons generated from progenitor cells.

Growth/differentiation factor-15/macrophage inhibitory cytokine-1 is a novel trophic factor for midbrain dopaminergic neurons in vivo

Transforming growth factor-betas (TGF-betas) constitute an expanding family of multifunctional cytokines with prominent roles in development, cell proliferation, differentiation, and repair. We have cloned, expressed, and raised antibodies against a distant member of the TGF-betas, growth/differentiation factor-15 (GDF-15). GDF-15 is identical to macrophage inhibitory cytokine-1 (MIC-1). GDF-15/MIC-1 mRNA and protein are widely distributed in the developing and adult CNS and peripheral nervous systems, including choroid plexus and CSF. GDF-15/MIC-1 is a potent survival promoting and protective factor for cultured and iron-intoxicated dopaminergic (DAergic) neurons cultured from the embryonic rat midbrain floor. The trophic effect of GDF-15/MIC-1 was not accompanied by an increase in cell proliferation and astroglial maturation, suggesting that GDF-15/MIC-1 probably acts directly on neurons. GDF-15/MIC-1 also protects 6-hydroxydopamine (6-OHDA)-lesioned nigrostriatal DAergic neurons in vivo. Unilateral injections of GDF-15/MIC-1 into the medial forebrain bundle just above the substantia nigra (SN) and into the left ventricle (20 microgram each) immediately before a 6-OHDA injection (8 microgram) prevented 6-OHDA-induced rotational behavior and significantly reduced losses of DAergic neurons in the SN. This protection was evident for at least 1 month. Administration of 5 microgram of GDF-15/MIC-1 in the same paradigm also provided significant neuroprotection. GDF-15/MIC-1 also promoted the serotonergic phenotype of cultured raphe neurons but did not support survival of rat motoneurons. Thus, GDF-15/MIC-1 is a novel neurotrophic factor with prominent effects on DAergic and serotonergic neurons. GDF-15/MIC-1 may therefore have a potential for the treatment of Parkinson's disease and disorders of the serotonergic system.

GDNF - a stranger in the TGF-beta superfamily?

Glial cell line-derived neurotrophic factor (GDNF) family, consisting of GDNF, neurturin, artemin and persephin are distant members of the transforming growth factor-beta (TGF-beta) superfamily. Unlike other members of the TGF-beta superfamily, which signal through the receptor serine-threonine kinases, GDNF family ligands activate intracellular signalling cascades via the receptor tyrosine kinase Ret. GDNF family ligands first bind to the glycosylphosphatidylinositol (GPI)-anchored GDNF family receptor alpha (GFRalpha) and then the GDNF family ligand-GFRalpha complex binds to and stimulates autophosphorylation of Ret. Alternatively, a preassociated complex between GFRalpha and Ret could form the binding site for the GDNF family ligand. GFRalpha1, GFRalpha2, GFRalpha3 and GFRalpha4 are the physiological coreceptors for GDNF, neurturin, artemin and persephin, respectively. Although all GDNF family ligands signal via activated Ret, GDNF can signal also via GFRalpha1 in the absence of Ret. GPI-anchored GFRalpha receptors are localized in plasma membrane to lipid rafts. GDNF binding to GFRalpha1 also recruits Ret to the lipid rafts and triggers association with Src, which is required for effective downstream signalling, leading to differentiation and neuronal survival. GDNF family ligands are potent survival factors for midbrain dopamine neurons, motoneurons, noradrenergic neurons, as well as for sympathetic, parasympathetic and sensory neurons. However, for most neuronal populations, except for motoneurons, TGF-beta is required as a cofactor for GDNF family ligand signalling. Because GDNF and neurturin can rescue dopamine neurons in the animal models of Parkinson disease, as well as motoneurons in vivo, hopes have been raised that GDNF family ligands may be new drugs for the treatment of neurodegenerative diseases. GDNF also has distinct functions outside the nervous system, promoting ureteric branching in kidney development and regulating spermatogenesis.

Functions of transforming growth factor-beta isoforms in the nervous system. Cues based on localization and experimental in vitro and in vivo evidence

This review briefly describes the cellular distribution and documented roles of the transforming growth factor (TGF)-beta isoforms TGF-beta2 and -beta3 in the central and peripheral nervous system. TGF-beta2 and -beta3 are coexpressed in developing radial glial and mature astroglial and Schwann cells, as well as in subpopulations of differentiated neurons, most prominently in cortical, hippocampal, and brainstem/spinal cord motor neurons. In vitro studies have suggested a number of potential, physiologically relevant functions for TGF-betas including regulation of astroglial cell proliferation, expression of adhesion molecules, survival promoting roles for neurons in combination with established neurotrophic factors, and differentiative actions on neurons.

The transforming growth factor-betas: structure, signaling, and roles in nervous system development and functions

Transforming growth factor-betas (TGF-betas) are among the most widespread and versatile cytokines. Here, we first provide a brief overview of their molecular biology, biochemistry, and signaling. We then review distribution and functions of the three mammalian TGF-beta isoforms, beta1, beta2, and beta3, and their receptors in the developing and adult nervous system. Roles of TGF-betas in the regulation of radial glia, astroglia, oligodendroglia, and microglia are addressed. Finally, we review the current state of knowledge concerning the roles of TGF-betas in controlling neuronal performances, including the regulation of proliferation of neuronal precursors, survival/death decisions, and neuronal differentiation.

Reduction of endogenous transforming growth factors beta prevents ontogenetic neuron death

We show that following immunoneutralization of endogenous transforming growth factors beta (TGF-beta) in the chick embryo, ontogenetic neuron death of ciliary, dorsal root and spinal motor neurons was largely prevented, and neuron losses following limb bud ablation were greatly reduced. Likewise, preventing TGF-beta signaling by treatment with a TbetaR-II fusion protein during the period of ontogenetic cell death in the ciliary ganglion rescued all neurons that normally die. TUNEL staining revealed decreased numbers of apoptotic cells following antibody treatment. Exogenous TGF-beta rescued the TGF-beta-deprived phenotype. We conclude that TGF-beta is critical in regulating ontogenetic neuron death as well as cell death following neuronal target deprivation.

Localization of transforming growth factor-beta1 and receptor mRNA after experimental spinal cord injury

Transforming growth factor-beta1 (TGFbeta1) is a cytokine/growth factor found within the pathological central nervous system. TGFbeta1 has been shown to inhibit the release of cytotoxic molecules from microglia and macrophages, decrease astrocyte proliferation, and promote neuron survival. Because of the relevance of these actions to spinal cord injury, we examined TGFbeta1 and its receptors betaRI and betaRII mRNA levels and localization within the contused rat spinal cord using in situ hybridization. At the lesion site, TGFbeta1 mRNA peaked at 7 days postinjury and declined thereafter. Temporal and spatial localization of the betaRI and betaRII receptor mRNA closely mimicked that for TGFbeta1 in the epicenter. TGFbeta1, betaRI, and betaRII mRNAs also were elevated rostral and caudal to the injury, especially in regions known to contain activated microglia and degenerating axon profiles. Immunohistochemical staining of nearby sections confirmed that the highest levels of TGFbeta1 and receptor mRNA corresponded to regions filled with activated microglia and macrophages. The similar expression pattern of TGFbeta1, betaRI, and betaRII mRNA within the injured spinal cord suggests a local site of action. Since TGFbeta1 can act as an immunosuppressant as well as a stimulant for growth factors and neurite sprouting, it likely plays an important role, both temporally and spatially, in orchestrating postinjury events within the spinal cord.

Apoptosis modulators in the therapy of neurodegenerative diseases

Apoptosis is a prerequisite to model the developing nervous system. However, an increased rate of cell death in the adult nervous system underlies neurodegenerative disease and is a hallmark of multiple sclerosis (MS) Alzheimer's- (AD), Parkinson- (PD), or Huntington's disease (HD). Cell surface receptors (e.g., CD95/APO-1/Fas; TNF receptor) and their ligands (CD95-L; TNF) as well as evolutionarily conserved mechanisms involving proteases, mitochondrial factors (e.g. , Bcl-2-related proteins, reactive oxygen species, mitochondrial membrane potential, opening of the permeability transition pore) or p53 participate in the modulation and execution of cell death. Effectors comprise oxidative stress, inflammatory processes, calcium toxicity and survival factor deficiency. Therapeutic agents are being developed to interfere with these events, thus conferring the potential to be neuroprotective. In this context, drugs with anti-oxidative properties, e.g., flupirtine, N-acetylcysteine, idebenone, melatonin, but also novel dopamine agonists (ropinirole and pramipexole) have been shown to protect neuronal cells from apoptosis and thus have been suggested for treating neurodegenerative disorders like AD or PD. Other agents like non-steroidal anti-inflammatory drugs (NSAIDs) partly inhibit cyclooxygenase (COX) expression, as well as having a positive influence on the clinical expression of AD. Distinct cytokines, growth factors and related drug candidates, e.g., nerve growth factor (NGF), or members of the transforming growth factor-beta (TGF-beta ) superfamily, like growth and differentiation factor 5 (GDF-5), are shown to protect tyrosine hydroxylase or dopaminergic neurones from apoptosis. Furthermore, peptidergic cerebrolysin has been found to support the survival of neurones in vitro and in vivo. Treatment with protease inhibitors are suggested as potential targets to prevent DNA fragmentation in dopaminergic neurones of PD patients. Finally, CRIB (cellular replacement by immunoisolatory biocapsule) is an auspicious gene therapeutical approach for human NGF secretion, which has been shown to protect cholinergic neurones from cell death when implanted in the brain. This review summarises and evaluates novel aspects of anti-apoptotic concepts and pharmacological intervention including gene therapeutical approaches currently being proposed or utilised to treat neurodegenerative diseases.

Expression of the GDNF family members and their receptors in the mature rat cochlea

The GDNF family comprises glial cell line-derived neurotrophic factor (GDNF) and the related proteins neurturin, artemin and persephin, which form a subgroup of the TGF-beta superfamily of growth factors. All four neurotrophic factors provide neuronal cell protection and cell survival. GDNF expression was found in the cochlea, and GDNF has been shown to be effective for inner ear protection from drugs and noise-induced insults. As the other members of the GDNF family also provide protective effects on neuronal cells, they may play important roles in the inner ear. We used RT-PCR to examine the expression of GDNF, neurturin, artemin, persephin and their receptors GFRalpha-1, GFRalpha-2, GFRalpha-3 and c-ret in whole rat cochlea as well as in functionally different subfractions (modiolus and sensorineural epithelium/lateral wall) and compared the levels of neurotrophin and receptor mRNAs in the cochlea to those in substantia nigra brain region. Our results demonstrate the expression of all GDNF family members and their receptors in cochlea and substantia nigra. However, the relative levels of mRNA were different for several genes tested in subfractions of the cochlea and/or compared to expression levels in substantia nigra. The presence of mRNA for all four members of the GDNF family and their preferred receptors in the rat cochlea suggests potential functional importance of these neurotrophic factors as protection and survival factors in the inner ear.

Co-activation of TGF-ss and cytokine signaling pathways are required for neurotrophic functions

This article summarizes and interprets recent data from our laboratories suggesting that transforming growth factor-ss (TGF-ss1, -ss2, -ss3) is essentially required, in vitro and in vivo, for the neurotrophic signaling of glial cell line-derived neurotrophic factor (GDNF). TGF-ss, which is synthesized by and released from neurons, also synergizes with neurotrophins and members of the neurokine and fibroblast growth factor families by increasing their efficacies. However, when applied to purified neuron populations without other factors being added, TGF-ss does not promote survival or differentiation. Together, these data suggest that neither TGF-ss nor GDNF fulfil essential criteria of a typical neurotrophic factor, as e.g. nerve growth factor (NGF). Moreover, the neurotrophic activity of NGF and other classic neurotrophic factors is apparently based, to a significant extent, on their co-operativity with TGF-ss. Mechanisms, by which TGF-ss generates neurotrophic effects and synergizes with other cytokines are beginning to emerge. Recruitment and/or stabilization of receptors and cross-talks at different levels of signal transduction are likely to be implied in generating the neurotrophic potential of the TGF-ss/cytokine synergisms. Together, these data outline a novel role of TGF-ss in a key event of nervous system development, ontogenetic neuron death. Conceptually more important, however, may be the broadening of the neurotrophic factor concept, which now has to imply the possibility that two cytokines, each being ineffective by itself, become neurotrophically active when acting in concert.

Mammalian-cell-produced neurturin (NTN) is more potent than purified Escherichia coli-produced NTN

Neurturin (NTN) is a recently identified homologue of glial-cell-line-derived neurotrophic factor. Both factors promote the survival of dopaminergic (DA) neurons. We investigated the biological activity of mammalian-cell-produced NTN versus purified Escherichia coli-produced NTN. Baby hamster kidney cells were engineered to stably secrete mature human NTN. Mammalian-cell-derived NTN enhanced the activity of embryonic DA neurons in vitro, with greater potency (maximum effect achieved in the picogram range) than purified E. coli-produced NTN. Cell-based delivery of NTN (less than 10 ng/day) was also shown to be biologically active in vivo. These results suggest that mammalian-cell-derived NTN, synthesized de novo and delivered in small quantities to the parenchyma at the target site, may be as active as much larger quantities of purified, E. coli-produced NTN, delivered by other means.

Other neurotrophic factors: glial cell line-derived neurotrophic factor (GDNF)

Glial cell line-derived neurotrophic factor (GDNF) was first discovered as a potent survival factor for midbrain dopaminergic neurons and was then shown to rescue these neurons in animal models of Parkinson's disease. GDNF is a more potent survival factor for dopaminergic neurons and the noradrenergic neurons of the locus coeruleus than other neurotrophic factors, and an almost 100 times more efficient survival factor for spinal motor neurons than the neurotrophins. The members of the GDNF family, GDNF, neurturin (NTN), persephin (PSP), and artemin (ART), have seven conserved cysteine residues with similar spacing, making them distant members of the transforming growth factor-beta (TGF-beta) superfamily. Like the members of the neurotrophin family, the GDNF-like growth factors belong structurally to the cysteine knot proteins. Like neurotrophins, GDNF family proteins are responsible for the development and maintenance of various sets of sensory and sympathetic neurons but, in addition, GDNF and NTN are also responsible for the development and survival of the enteric neurons, and NTN for parasympathetic neurons. All neurotrophins bind to the p75 low-affinity receptor, but their ligand specificity is determined by trk receptor tyrosine kinases. GDNF, NTN, PSP, and ART mediate their signals via a common receptor tyrosine kinase, Ret, but their ligand specificity is determined by a novel class of glycosylphosphatidylinositol (GPI)-anchored proteins called the GDNF family receptor alpha (GFR alpha). GDNF binds preferentially to GFR alpha1, NTN GFR alpha2, ART GRF alpha3, and PSP GFR alpha4 as a co-receptor to activate Ret. GFR alpha4 has until now been described only from chicken. Although the GDNF family members signal mainly via Ret receptor tyrosine kinase, there is recent evidence that they can also mediate their signals via GFR alpha receptors independently of Ret. The GDNF family of growth factors, unlike neurotrophins, has a well-defined function outside the nervous system. Recent transgenic and organ culture experiments have clearly demonstrated that GDNF is a mesenchyme-derived signaling molecule for the promotion of ureteric branching in kidney development. NTN, ART, and PSP are also expressed in the developing kidney, and NTN and PSP induce ureteric branching in vitro, but their true in vivo role in kidney morphogenesis is still unclear.