Growth/differentiation factor 15 controls ependymal and stem cell number in the V-SVZ.

The expression of growth/differentiation factor (GDF) 15 increases in the ganglionic eminence (GE) late in neural development, especially in neural stem cells (NSCs). However, GDF15 function in this region remains unknown. We report that GDF15 receptor is expressed apically in the GE and that GDF15 ablation promotes proliferation and cell division in the embryonic GE and in the adult ventricular-subventricular zone (V-SVZ). This causes a transient generation of additional neuronal progenitors, compensated by cell death, and a lasting increase in the number of ependymal cells and apical NSCs. Finally, both GDF15 receptor and the epidermal growth factor receptor (EGFR) were expressed in progenitors and mutation of GDF15 affected EGFR signaling. However, only exposure to exogenous GDF15, but not to EGF, normalized proliferation and the number of apical progenitors. Thus, GDF15 regulates proliferation of apical progenitors in the GE, thereby affecting the number of ependymal cells and NSCs.

Intratumoral platelet aggregate formation in a murine preclinical glioma model depends on podoplanin expression on tumor cells.

Binding of the sialomucin-like transmembrane glycoprotein podoplanin (PDPN) to the platelet receptor C-type lectin-like receptor 2 induces platelet activation and aggregation. In human high-grade gliomas, PDPN is highly expressed both in tumor cells and in tumor-associated astrocytes. In glioma patients, high expression of PDPN is associated with worse prognosis and has been shown to correlate with intratumoral platelet aggregation and an increased risk of venous thromboembolism (VTE). To functionally assess the role of PDPN in platelet aggregation in vivo, we established a syngeneic orthotopic murine glioma model in C57/Bl6 mice, based on transplantation of p53- and Pten-deficient neural stem cells. This model is characterized by the presence of intratumoral platelet aggregates and by the upregulation of PDPN both in glioma cells and in astrocytes, reflecting the characteristics of human gliomas. Deletion of PDPN either in tumor cells or in astrocytes resulted in glioma formation with similar penetrance and grade compared with control mice. Importantly, only the lack of PDPN in tumor cells, but not in astrocytes, caused a significant reduction in intratumoral platelet aggregates, whereas in vitro, both cell types have similar platelet aggregation-inducing capacities. Our results demonstrate a causative link between PDPN and platelet aggregation in gliomas and pinpoint the tumor cells as the major players in PDPN-induced platelet aggregation. Our data indicate that blocking PDPN specifically on tumor cells could represent a novel strategy to prevent platelet aggregation and thereby reduce the risk of VTE in glioma patients.

An advanced glioma cell invasion assay based on organotypic brain slice cultures.

BACKGROUND: The poor prognosis for glioblastoma patients is caused by the diffuse infiltrative growth pattern of the tumor. Therefore, the molecular and cellular processes underlying cell migration continue to be a major focus of glioblastoma research. Emerging evidence supports the concept that the tumor microenvironment has a profound influence on the functional properties of tumor cells. Accordingly, substantial effort must be devoted to move from traditional two-dimensional migration assays to three-dimensional systems that more faithfully recapitulate the complex in vivo tumor microenvironment. METHODS: In order to mimic the tumor microenvironment of adult gliomas, we used adult organotypic brain slices as an invasion matrix for implanted, fluorescently labeled tumor spheroids. Cell invasion was imaged by confocal or epi-fluorescence microscopy and quantified by determining the average cumulative sprout length per spheroid. The tumor microenvironment was manipulated by treatment of the slice with small molecule inhibitors or using different genetically engineered mouse models as donors. RESULTS: Both epi-fluorescence and confocal microscopy were applied to precisely quantify cell invasion in this ex vivo approach. Usage of a red-emitting membrane dye in addition to tissue clearing drastically improved epi-fluorescence imaging. Preparation of brain slices from of a genetically engineered mouse with a loss of a specific cell surface protein resulted in significantly impaired tumor cell invasion. Furthermore, jasplakinolide treatment of either tumor cells or brain slice significantly reduced tumor cell invasion. CONCLUSION: We present an optimized invasion assay that closely reflects in vivo invasion by the implantation of glioma cells into organotypic adult brain slice cultures with a preserved cytoarchitecture. The diversity of applications including manipulation of the tumor cells as well as the microenvironment, permits the investigation of rate limiting factors of cell migration in a reliable context. This model will be a valuable tool for the discovery of the molecular mechanisms underlying glioma cell invasion and, ultimately, the development of novel therapeutic strategies.

Growth/differentiation factor 15 promotes EGFR signalling, and regulates proliferation and migration in the hippocampus of neonatal and young adult mice.

The activation of epidermal growth factor receptor (EGFR) affects multiple aspects of neural precursor behaviour, including proliferation and migration. Telencephalic precursors acquire EGF responsiveness and upregulate EGFR expression at late stages of development. The events regulating this process and its significance are still unclear. We here show that in the developing and postnatal hippocampus (HP), growth/differentiation factor (GDF) 15 and EGFR are co-expressed in primitive precursors as well as in more differentiated cells. We also provide evidence that GDF15 promotes responsiveness to EGF and EGFR expression in hippocampal precursors through a mechanism that requires active CXC chemokine receptor (CXCR) 4. Besides EGFR expression, GDF15 ablation also leads to decreased proliferation and migration. In particular, lack of GDF15 impairs both processes in the cornu ammonis (CA) 1 and only proliferation in the dentate gyrus (DG). Importantly, migration and proliferation in the mutant HP were altered only perinatally, when EGFR expression was also affected. These data suggest that GDF15 regulates migration and proliferation by promoting EGFR signalling in the perinatal HP and represent a first description of a functional role for GDF15 in the developing telencephalon.

The murine growth differentiation factor 15 is not essential for systemic iron homeostasis in phlebotomized mice.

In conditions of increased erythropoiesis, expression of hepcidin, the master regulator of systemic iron homeostasis, is decreased to allow for the release of iron into the blood stream from duodenal enterocytes and macrophages. It has been suggested that hepcidin suppression is controlled by growth differentiation factor 15 (GDF15), a member of the transforming growth factor-ß superfamily of cytokines that is secreted from developing erythroblasts. In this study, we analyzed iron-related parameters in mice deficient for GDF15 under steady-state conditions and in response to increased erythropoietic activity induced by blood loss. We demonstrate that GDF15 suppresses the hepatic mRNA expression of some BMP/TGFß target genes but not of hepcidin, and show that GDF15 is not required to balance iron homeostasis in response to blood loss.

Growth differentiation factor-15 deficiency inhibits atherosclerosis progression by regulating interleukin-6-dependent inflammatory response to vascular injury.

BACKGROUND: Growth differentiation factor (GDF)-15 is a distant and divergent member of the transforming growth factor-ß superfamily (TGF-ß) . There is growing evidence indicating the involvement of GDF-15 in various pathologies. Expression of GDF-15 is induced under conditions of inflammation and increased GDF-15 serum levels are suggested as a risk factor for cardiovascular diseases. METHODS AND RESULTS: We show here that GDF-15 and proinflammatory cytokine interleukin (IL)-6 levels are highly increased (5-fold) in cultured oxidized low-density lipoproteins-stimulated peritoneal macrophages derived from GDF-15(+/+)/apolipoprotein (apo) E(-/-), mice. Notably, IL-6 induction on oxidized low-density lipoproteins stimulation is completely abolished in the absence of GDF-15. Consistent with our in vitro data GDF-15 mRNA expression and protein levels are upregulated (2.5- to 6-fold) in the atherosclerotic vessel wall of GDF-15(+/+)/apoE(-/-) mice after a cholesterol-enriched diet. GDF-15 deficiency inhibits lumen stenosis (52%) and (18)FDG uptake (34%) in the aortic arch despite increased serum triglyceride/cholesterol levels and elevated body weight. Immunohistomorphometric investigations of atherosclerotic lesions reveal a decreased percentage of inflammatory CD11b(+) (57%) or IL-6(+), leukocytes, and apoptotic cells (74%) after 20 weeks. However, the total number of macrophages and cell density in atherosclerotic lesions of the innominate artery are increased in GDF-15(-/-)/apoE(-/-) mice. CONCLUSIONS: Our data suggest that GDF-15 is involved in orchestrating atherosclerotic lesion progression by regulating apoptotic cell death and IL-6-dependent inflammatory responses to vascular injury.

Regulation of GDF-15, a distant TGF-ß superfamily member, in a mouse model of cerebral ischemia.

GDF-15 is a novel distant member of the TGF-ß superfamily and is widely distributed in the brain and peripheral nervous system. We have previously reported that GDF-15 is a potent neurotrophic factor for lesioned dopaminergic neurons in the substantia nigra, and that GDF-15-deficient mice show progressive postnatal losses of motor and sensory neurons. We have now investigated the regulation of GDF-15 mRNA and immunoreactivity in the murine hippocampal formation and selected cortical areas following an ischemic lesion by occlusion of the middle cerebral artery (MCAO). MCAO prominently upregulates GDF-15 mRNA in the hippocampus and parietal cortex at 3 h and 24 h after lesion. GDF-15 immunoreactivity, which is hardly detectable in the unlesioned brain, is drastically upregulated in neurons identified by double-staining with NeuN. NeuN staining reveals that most, if not all, neurons in the granular layer of the dentate gyrus and pyramidal layers of the cornu ammonis become GDF-15-immunoreactive. Moderate induction of GDF-15 immunoreactivity has been observed in a small number of microglial cells identified by labeling with tomato lectin, whereas astroglial cells remain GDF-15-negative after MCAO. Comparative analysis of the size of the infarcted area after MCAO in GDF-15 wild-type and knockout mice has failed to reveal significant differences. Together, our data substantiate the notion that GDF-15 is prominently upregulated in the lesioned brain and might be involved in orchestrating post-lesional responses other than the trophic support of neurons.

Progressive postnatal motoneuron loss in mice lacking GDF-15.

Growth/differentiation factor-15 (GDF-15) is a widely expressed distant member of the TGF-beta superfamily with prominent neurotrophic effects on midbrain dopaminergic neurons. We show here that GDF-15-deficient mice exhibit progressive postnatal losses of spinal, facial, and trigeminal motoneurons. This deficit reaches a approximately 20% maximum at 6 months and is accompanied by losses of motor axons and significant impairment of rotarod skills. Similarly, sensory neurons in dorsal root ganglia (L4, L5) are reduced by 20%, whereas sympathetic neurons are not affected. GDF-15 is expressed and secreted by Schwann cells, retrogradely transported along adult sciatic nerve axons, and promotes survival of axotomized facial neurons as well as cultured motor, sensory, and sympathetic neurons. Despite striking similarities in the GDF-15 and CNTF knock-out phenotypes, expression levels of CNTF and other neurotrophic factors in the sciatic nerve were unaltered suggesting that GDF-15 is a genuine novel trophic factor for motor and sensory neurons.

Expression and putative functions of GDF-15, a member of the TGF-beta superfamily, in human glioma and glioblastoma cell lines.

Recent studies have demonstrated growth-inhibiting effects of growth differentiation factor-15 (GDF-15) on different cancer cell lines invitro and on tumor growth in vivo. Here, we present data concerning expression of GDF-15 in glioblastoma. We found low levels of GDF-15 transcripts in primary glioblastoma. Thus, GDF-15 expression might be exploited as a useful indicator for distinguishing primary from other glial derived tumors. In contrast to the documented proapoptotic and anti-tumorigenic activities of GDF-15 in several cancer cell lines, our data suggest that GDF-15 does not decrease proliferation of glioblastoma cell lines, while its effects on invasiveness are not consistent.

GDNF prevents TGF-beta-induced damage of the plasma membrane in cerebellar granule neurons by suppressing activation of p38-MAPK via the phosphatidylinositol 3-kinase pathway.

Transforming growth factor-beta (TGF-beta) and glial-cell-line-derived neurotrophic factor (GDNF) have been shown to synergize in several paradigms of neuronal survival. We have previously shown that cerebellar granule neurons (CGN) degenerate in low potassium via ERK1/2 (extra-cellular-regulated kinase)-dependent plasma membrane (PM) damage and caspase-3-dependent DNA fragmentation. Here, we have investigated the putative synergistic function of GDNF and TGF-beta in CGN degeneration. GDNF alone prevents low-potassium-induced caspase-3 activation and DNA fragmentation but does not affect either low-potassium-induced ERK activation or PM damage. TGF-beta alone does not affect low-potassium-induced DNA fragmentation but potentiates low-potassium-induced PM damage. This effect of TGF-beta is independent of ERK1/2 activation but dependent on p38-MAPK (mitogen-activated protein kinase) activation. When co-applied with TGF-beta, GDNF paradoxically antagonizes TGF-beta-induced potentiation of PM damage by inhibiting TGF-beta-induced p38-MAPK activation. In addition, PI3K (phosphatidylinositol 3-kinase) inhibitors abolish the GDNF effect. This study thus demonstrates a differential mechanism of action of GDNF and TGF-beta on CGN degeneration. GDNF inhibits caspase-3-dependent DNA fragmentation but does not affect ERK-dependent PM damage. However, GDNF can attenuate TGF-beta-induced p38-MAPK-dependent PM damage via the PI3K pathway.

Specificity in the crosstalk of TGFbeta/GDNF family members is determined by distinct GFR alpha receptors.

Glial cell line-derived neurotrophic factor (GDNF) and neurturin (NRTN) are neurotrophic factors for parasympathetic neurons including ciliary ganglion (CG) neurons. Recently, we have shown that survival and signaling mediated by GDNF in CG neurons essentially requires transforming growth factor beta (TGFbeta). We have provided evidence that TGFbeta regulates the availability of the glycosyl phosphatidylinositol (GPI)-anchored GDNF receptor alpha 1 (GFRalpha1) by promoting the recruitment of the receptor to the plasma membrane. We report now that in addition to GDNF, NRTN, but not persephin (PSPN) or artemin (ARTN), is able to promote survival of CG neurons. Interestingly, in contrast to GDNF, NRTN is not dependent on cooperation with TGFbeta, but efficiently promotes neuronal survival and intracellular signaling in the absence of TGFbeta. Additional treatment with TGFbeta does not further increase the NRTN response. Both NRTN and GDNF exclusively bind to and activate their cognate receptors, GFRalpha2 and GFRalpha1, respectively, as shown by the use of receptor-specific neutralizing antibodies. Immunocytochemical staining for the two receptors on the surface of CG neurons reveals that, in contrast to the effect on GFRalpha1, TGFbeta is not required for recruitment of GFRalpha2 to the plasma membrane. Moreover, binding of radioactively labeled GDNF but not NRTN is increased upon treatment of CG neurons with TGFbeta. Disruption of TGFbeta signaling does interfere with GDNF-, but not NRTN-mediated signaling and survival. We propose a model taking into account data from GFRalpha1 crystallization and ontogenetic development of the CG that may explain the differences in TGFbeta-dependence of GDNF and NRTN.

The transforming growth factor-beta superfamily member growth-differentiation factor-15 protects the heart from ischemia/reperfusion injury.

Data from the Women's Health Study show that serum levels of growth-differentiation factor-15 (GDF-15), a distant member of the transforming growth factor-beta superfamily, are an independent risk indicator for adverse cardiovascular events. However, the cellular sources, upstream regulators, and functional effects of GDF-15 in the cardiovascular system have not been elucidated. We have identified GDF-15 by cDNA expression array analysis as a gene that is strongly upregulated by nitrosative stress in cultured cardiomyocytes isolated from 1- to 3-day-old rats. GDF-15 mRNA and pro-peptide expression levels were also induced in cardiomyocytes subjected to simulated ischemia/reperfusion (I/R) via NO-peroxynitrite-dependent signaling pathways. GDF-15 was actively secreted into the culture supernatant, suggesting that it might exert autocrine/paracrine effects during I/R. To explore the in vivo relevance of these findings, mice were subjected to transient or permanent coronary artery ligation. Myocardial GDF-15 mRNA and pro-peptide abundance rapidly increased in the area-at-risk after ischemic injury. Similarly, patients with an acute myocardial infarction had enhanced myocardial GDF-15 pro-peptide expression levels. As shown by immunohistochemistry, cardiomyocytes in the ischemic area contributed significantly to the induction of GDF-15 in the infarcted human heart. To delineate the function of GDF-15 during I/R, Gdf-15 gene-targeted mice were subjected to transient coronary artery ligation for 1 hour followed by reperfusion for 24 hours. Gdf-15-deficient mice developed greater infarct sizes and displayed more cardiomyocyte apoptosis in the infarct border zone after I/R compared with wild-type littermates, indicating that endogenous GDF-15 limits myocardial tissue damage in vivo. Moreover, treatment with recombinant GDF-15 protected cultured cardiomyocytes from apoptosis during simulated I/R as shown by histone ELISA, TUNEL/Hoechst staining, and annexin V/propidium iodide fluorescence-activated cell sorting (FACS) analysis. Mechanistically, the prosurvival effects of GDF-15 in cultured cardiomyocytes were abolished by phosphoinositide 3-OH kinase inhibitors and adenoviral expression of dominant-negative Akt1 (K179M mutation). In conclusion, our study identifies induction of GDF-15 in the heart as a novel defense mechanism that protects from I/R injury.

Insulin-like growth factor 1 inhibits extracellular signal-regulated kinase to promote neuronal survival via the phosphatidylinositol 3-kinase/protein kinase A/c-Raf pathway.

Extracellular signal-regulated kinase (ERK) activation has been shown to promote neuronal death in various paradigms. We demonstrated previously that the late and sustained ERK activation in cerebellar granule neurons (CGNs) cultured in low potassium predominantly promotes plasma membrane (PM) damage. Here, we examined the effects of a well established neuronal survival factor, insulin-like growth factor 1 (IGF-1), on the ERK cell death pathway. Stimulation of CGNs with IGF-1 induced an early and transient ERK activation but abrogated the appearance of late and sustained ERK. Withdrawal or readdition of IGF-1 after 4 h in low potassium failed to prevent sustained ERK activation and cell death. IGF-1 activated the protein kinase A (PKA) to mediate ERK inhibition via c-Raf phosphorylation at an inhibitory site (Ser259). Phosphatidylinositol 3-kinase (PI3K) or PKA inhibitors, but not a specific Akt inhibitor, abrogated PKA signaling. This suggests that the PI3K/PKA/c-Raf-Ser259 pathway mediates ERK inhibition by IGF-1 independent of Akt. In addition, adenoviral-mediated expression of constitutively active MEK (mitogen-activated protein kinase kinase) or Sindbis viral-mediated expression of mutant Raf Ser259Ala both attenuated IGF-1-mediated prevention of PM damage. Activation of caspase-3 promoted DNA damage. Its inhibition by IGF-1 was both PI3K and Akt dependent but PKA independent. 8-Br-cAMP, an activator of PKA, induced phosphorylation of c-Raf-Ser259 and inhibited ERK activation without affecting caspase-3. This indicates a selective role for PKA in ERK inhibition through c-Raf-Ser259 phosphorylation. Together, these data demonstrate that IGF-1 can positively and negatively regulate the ERK pathway in the same neuronal cell, and provide new insights into the PI3K/Akt/PKA signaling pathways in IGF-1-mediated neuronal survival.

Involvement of growth differentiation factor-15/macrophage inhibitory cytokine-1 (GDF-15/MIC-1) in oxLDL-induced apoptosis of human macrophages in vitro and in arteriosclerotic lesions.

Growth differentiation factor-15/macrophage inhibitory cytokine-1 (GDF-15/MIC-1) is a new member of the transforming growth factor beta (TGF-beta) superfamily, which has most recently been found in activated macrophages (MPhi). We have now investigated GDF-15/MIC-1 in human MPhi after exposure to oxidized low-density lipoproteins (oxLDL) related mediators in vitro and in arteriosclerotic carotid arteries. Using RT-PCR and Western blotting a pronounced induction of GDF-15/MIC-1 expression by oxLDL, C6-ceramide, tumor necrosis factor (TNFalpha) and hydrogen peroxide (H2O2) was found in cultured human MPhi. In 11 human arteriosclerotic carotid arteries, immunohistochemical analyses supported by computer-assisted morphometry and regression analyses demonstrated a significant colocalization of GDF-15/MIC-1 immunoreactivity (IR) with oxLDL IR and manganese superoxide dismutase (MnSOD) IR in CD68 immunoreactive (ir) MPhi, which were also expressing AIF-IR (apoptosis-inducing factor), caspase-3-IR (CPP32), PARP-IR, c-Jun/AP-1-IR and p53-IR. Our data suggest that GDF-15/MIC-1 is inducible in human MPhi by oxLDL and its mediators in vitro and is supposed to contribute to oxidative stress dependent consequences in arteriosclerotic plaques, e.g. modulating apoptosis and inflammatory processes in activated MPhi.

ERK activation promotes neuronal degeneration predominantly through plasma membrane damage and independently of caspase-3.

Our recent studies have shown that extracellular-regulated protein kinase (ERK) promotes cell death in cerebellar granule neurons (CGN) cultured in low potassium. Here we report that the "death" phenotypes of CGN after potassium withdrawal are heterogeneous, allowing the distinction between plasma membrane (PM)-, DNA-, and PM/DNA-damaged populations. These damaged neurons display nuclear condensation that precedes PM or DNA damage. Inhibition of ERK activation either by U0126 or by dominant-negative mitogen-activated protein kinase/ERK kinase (MEK) overexpression results in a dramatic reduction of PM damaged neurons and nuclear condensation. In contrast, overexpression of constitutively active MEK potentiates PM damage and nuclear condensation. ERK-promoted cellular damage is independent of caspase-3. Persistent active ERK translocates to the nucleus, whereas caspase-3 remains in the cytoplasm. Antioxidants that reduced ERK activation and PM damage showed no effect on caspase-3 activation or DNA damage. These data identify ERK as an important executor of neuronal damage involving a caspase-3-independent mechanism.

Growth differentiation factor-15 prevents low potassium-induced cell death of cerebellar granule neurons by differential regulation of Akt and ERK pathways.

Growth differentiation factor-15 (GDF-15) is a novel member of the transforming growth factor-beta superfamily and has been shown to be induced in neurons subsequent to lesions. We have therefore begun to study its putative role in the regulation of neuron survival and apoptosis. Cultured cerebellar granule neurons (CGN) survive when maintained in high K(+) (25 mm) but undergo apoptosis when switched to low K(+) (5 mm). GDF-15 prevented death of CGN in low K(+). This effect could be blocked by phosphatidylinositol 3-kinase/Akt pathway inhibitors LY294002 or wortmannin. In contrast, mitogen-activated protein kinase (MEK)/extracellular-signal-regulated kinase (ERK) pathway inhibitors U0126 and PD98059 potentiated GDF-15 mediated survival and prevented cell death in low K(+) even without factor treatment. Immunoblots revealed GDF-15-induced phosphorylation of Akt and glycogen synthase kinase-3beta. This activation was suppressed by phosphatidylinositol 3-kinase inhibitors. Low K(+) induced delayed and persistent ERK activation, which was blocked by MEK inhibitors or GDF-15. ERK activation induced c-Jun, a member of the AP-1 transcription factor family. GDF-15 or U0126 prevented c-Jun activation. Furthermore, we show that GDF-15 prevented generation of reactive oxygen species, a known activator of ERK. Together, our data suggest that GDF-15 prevents apoptosis in CGN by activating Akt and inhibiting endogenously active ERK.

TGF-beta induces cell death in the oligodendroglial cell line OLI-neu.

We have shown that TGF-beta plays an important role during the period of developmental cell death in the nervous system. Immunoneutralization of TGF-beta prevents ontogenetic neuron death in vivo. Like neurons, oligodendrocytes are generated in excess and eliminated by apoptosis. It has been shown that oligodendrocyte progenitors and newly formed oligodendrocytes are especially susceptible to apoptosis. We choose the oligodendrocyte precursor cell line OLI-neu to address the question if TGF-beta could play a role for the control of oligodendrocyte proliferation and cell death. Flow cytometric analysis revealed that OLI-neu cells arrested in the G1 phase of the cell cycle underwent apoptosis in response to TGF-beta. TUNEL assays, apoptosis ELISA, and caspase assays substantiated the finding that OLI-neu cells died after TGF-beta treatment. Cell death could be inhibited by application of pan-caspase or caspase 8 and 9 inhibitors, whereas the inhibition of calpain was unaffected. Furthermore, we found a reduction of bcl-X(L) at the protein as well as at the mRNA level, while p27 was upregulated. The Smad cascade was activated while TGF-beta reduced the activity of the p42/p44 MAP kinase pathway. Together, these data show that TGF-beta induced apoptotic cell death in cells of oligodendroglial origin, whereby the signaling cascade involved the downregulation of antiapoptotic signaling such as bcl-X(L) leading to the activation of caspases.

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.