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.

Lineage and stage specific requirement for Dicer1 in sympathetic ganglia and adrenal medulla formation and maintenance.

The development of sympathetic neurons and chromaffin cells is differentially controlled at distinct stages by various extrinsic and intrinsic signals. Here we use conditional deletion of Dicer1 in neural crest cells and noradrenergic neuroblasts to identify stage specific functions in sympathoadrenal lineages. Conditional Dicer1 knockout in neural crest cells of Dicer1(Wnt1Cre) mice results in a rapid reduction in the size of developing sympathetic ganglia and adrenal medulla. In contrast, Dicer1 elimination in noradrenergic neuroblasts of Dicer1(DbhiCre) animals affects sympathetic neuron survival starting at late embryonic stages and chromaffin cells persist at least until postnatal week 1. A differential function of Dicer1 signaling for the development of embryonic noradrenergic and cholinergic sympathetic neurons is demonstrated by the selective increase in the expression of Tlx3 and the cholinergic marker genes VAChT and ChAT at E16.5. The number of Dbh, Th and TrkA expressing noradrenergic neurons is strongly decreased in Dicer1-deficient sympathetic ganglia at birth, whereas Tlx3(+)/ Ret(+) cholinergic neurons cells are spared from cell death. The postnatal death of chromaffin cells is preceded by the loss of Ascl1, mir-375 and Pnmt and an increase in the markers Ret and NF-M, which suggests that Dicer1 is required for the maintenance of chromaffin cell differentiation and survival. Taken together, these findings demonstrate distinct stage and lineage specific functions of Dicer1 signaling in differentiation and survival of sympathetic neurons and adrenal chromaffin cells.

Growth/differentiation factor-15 deficiency compromises dopaminergic neuron survival and microglial response in the 6-hydroxydopamine mouse model of Parkinson's disease.

Growth/differentiation factor-15 (Gdf-15) is a member of the TGF-ß superfamily and a pleiotropic, widely distributed cytokine, which has been shown to play roles in various pathologies, including inflammation. Analysis of Gdf-15(-/-) mice has revealed that it serves the postnatal maintenance of spinal cord motor neurons and sensory neurons. In a previous study, exogenous Gdf-15 rescued 6-hydroxydopamine (6-OHDA) lesioned Gdf-15(+/+) nigrostriatal dopaminergic (DAergic) neurons in vitro and in vivo. Whether endogenous Gdf-15 serves the physiological maintenance of nigrostriatal DAergic neurons in health and disease is not known and was addressed in the present study. Stereotactic injection of 6-OHDA into the medial forebrain bundle (MFB) led to a significant decline in the numbers of DAergic neurons in both Gdf-15(+/+) and Gdf-15(-/-) mice over a time-period of 14days. However, this decrease was exacerbated in the Gdf-15(-/-) mice, with only 5.5% surviving neurons as compared to 24% in the Gdf-15(+/+) mice. Furthermore, the microglial response to the 6-OHDA lesion was reduced in Gdf-15(-/-) mice, with significantly lower numbers of total and activated microglia and a differential cytokine expression as compared to the Gdf-15(+/+) mice. Using in vitro models, we could demonstrate the importance of endogenous Gdf-15 in promoting DAergic neuron survival thus highlighting its relevance in a direct neurotrophic supportive role. Taken together, these results indicate the importance of Gdf-15 in promoting survival of DAergic neurons and regulating the inflammatory response post 6-OHDA lesion.

Gdf-15 deficiency does not alter vulnerability of nigrostriatal dopaminergic system in MPTP-intoxicated mice.

Growth/differentiation factor-15 (Gdf-15) is a member of the transforming growth factor-ß (Tgf-ß) superfamily and has been shown to be a potent neurotrophic factor for midbrain dopaminergic (DAergic) neurons both in vitro and in vivo. Gdf-15 has also been shown to be involved in inflammatory processes. The aim of this study was to identify the role of endogenous Gdf-15 in the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) mouse model of Parkinson's disease (PD) by comparing Gdf-15 (+/+) and Gdf-15 (-/-) mice. At 4 days and 14 days post-MPTP administration, both Gdf-15 (+/+) and Gdf-15 (-/-) mice showed a similar decline in DAergic neuron numbers and in striatal dopamine (DA) levels. This was followed by a comparable restorative phase at 90 days and 120 days, indicating that the absence of Gdf-15 does not affect the susceptibility or the recovery capacity of the nigrostriatal system after MPTP administration. The MPTP-induced microglial and astrocytic response was not significantly altered between the two genotypes. However, pro-inflammatory and anti-inflammatory cytokine profiling revealed the differential expression of markers in Gdf-15 (+/+) and Gdf-15 (-/-) mice after MPTP administration. Thus, the MPTP mouse model fails to uncover a major role of endogenous Gdf-15 in the protection of MPTP-lesioned nigrostriatal DAergic neurons, in contrast to its capacity to protect the 6-hydroxydopamine-intoxicated nigrostriatal system.

MiR-124 is differentially expressed in derivatives of the sympathoadrenal cell lineage and promotes neurite elongation in chromaffin cells.

The neural-crest-derived sympathoadrenal cell lineage gives rise to sympathetic neurons and to endocrine chromaffin cells of the adrenal medulla. Both cell types express a largely overlapping set of genes, including those coding for the molecular machinery related to the synthesis and exocytotic release of catecholamines. During their early development, sympathetic neurons and chromaffin cells rely on a shared transcription factor network that controls the establishment of these common features. Despite many similarities, mature sympathetic neurons and chromaffin cells significantly differ regarding their morphology and function. Most prominently, sympathetic neurons possess axons that are absent in mammalian adrenal chromaffin cells. The molecular mechanism underlying the divergent development of sympathoadrenal cells into neuronal and endocrine cells remains elusive. Mutational inactivation of the ribonuclease dicer hints at the importance of microRNAs in this diversification. We show here that miR-124 is detectable in developing sympathetic neurons but absent in chromaffin cell precursors. We further demonstrate that miR-124 promotes neurite elongation when transfected into cultured chromaffin cells indicating its capability to support the establishment of a neuronal morphology in non-neuronal sympathoadrenal cells. Our results also show that treatment of PC12 cells with the neurotrophin nerve growth factor leads to an upregulation of miR-124 expression and that inhibition of miR-124 reduces nerve-growth-factor-induced neurite outgrowth in PC12 cells. Thus, our data indicate that miR-124 contributes to the establishment of specific neuronal features in developing sympathoadrenal cells.

Mir-592 regulates the induction and cell death-promoting activity of p75NTR in neuronal ischemic injury.

The neurotrophin receptor p75(NTR) has been implicated in mediating neuronal apoptosis after injury to the CNS. Despite its frequent induction in pathologic states, there is limited understanding of the mechanisms that regulate p75(NTR) expression after injury. Here, we show that after focal cerebral ischemia in vivo or oxygen-glucose deprivation in organotypic hippocampal slices or neurons, p75(NTR) is rapidly induced. A concomitant induction of proNGF, a ligand for p75(NTR), is also observed. Induction of this ligand/receptor system is pathologically relevant, as a decrease in apoptosis, after oxygen-glucose deprivation, is observed in hippocampal neurons or slices after delivery of function-blocking antibodies to p75(NTR) or proNGF and in p75(NTR) and ngf haploinsufficient slices. Furthermore, a significant decrease in infarct volume was noted in p75(NTR)-/- mice compared with the wild type. We also investigated the regulatory mechanisms that lead to post-ischemic induction of p75(NTR). We demonstrate that induction of p75(NTR) after ischemic injury is independent of transcription but requires active translation. Basal levels of p75(NTR) in neurons are maintained in part by the expression of microRNA miR-592, and an inverse correlation is seen between miR-592 and p75(NTR) levels in the adult brain. After cerebral ischemia, miR-592 levels fall, with a corresponding increase in p75(NTR) levels. Importantly, overexpression of miR-592 in neurons decreases the level of ischemic injury-induced p75(NTR) and attenuates activation of pro-apoptotic signaling and cell death. These results identify miR-592 as a key regulator of p75(NTR) expression and point to a potential therapeutic candidate to limit neuronal apoptosis after ischemic injury.

Growth/differentiation factor-15 and its role in peripheral nervous system lesion and regeneration.

Growth/differentiation factor-15 (GDF-15) is a distant member of the transforming growth factor beta (TGF-ß) superfamily. It is widely distributed in the nervous system, where it has been shown to play an important role in neuronal maintenance. The present study investigates the role of endogenous GDF-15 in sciatic nerve (SN) lesions using wild-type (WT) and GDF-15 knock-out (KO) mice. SN of 5-6-month-old mice were crushed or transected. Dorsal root ganglia (DRG) and nerve tissue were analyzed at different time points from 6 h to 9 weeks post-lesion. Both crush and transection induced GDF-15 mRNA and protein in the distal portion of the nerve, with a peak at day 7. DRG neuron death did not significantly differ between the genotypes; similarly, remyelination of regenerating axons was not affected by the genotype. Alternative macrophage activation and macrophage recruitment were more pronounced in the KO nerve. Protrusion speed of axons was similar in the two genotypes but WT axons showed better maturation, as indicated by larger caliber at 9 weeks. Furthermore, the regenerated WT nerve showed better performance in the electromyography test, indicating better functional recovery. We conclude that endogenous GDF-15 is beneficial for axon regeneration following SN crush.

Neurotrophic molecules in the treatment of neurodegenerative disease with focus on the retina: status and perspectives.

Neurotrophic factors are operationally defined as molecules that promote the survival and differentiation of neurons. Chemically, they belong to divergent classes of molecules but most of the classic neurotrophic factors are proteins. Together with stem cells, viral vectors and genetically engineered cells, they constitute important tools in neuroprotective and regenerative neurobiology. Protein neurotrophic molecules signal through receptors located on the cell membrane. Their downstream signaling exploits pathways that are often common to chemically different factors and frequently target a relatively restricted set of transcription factors, RNA interference and diverse molecular machinery involved in the life vs. death decisions of neurons. Application of neurotrophic factors with the aim of curing or, at least, improving the outcome of neurodegenerative diseases requires (1) profound knowledge of the complex molecular pathology of the disease, (2) the development of animal models as closely as possible resembling the human disease, (3) the identification of target cells to be addressed, (4) intense efforts in chemical engineering to ensure the stability of molecules or to design carriers and small analogs with the ability to cross the blood-brain barrier and (5) scrutinity with regard to possible side effects. Last, but not least, engineering efforts to optimize administration, e.g., by designing the right canulae and infusion devices, are important for the successful translation of preclinical advances into clinical benefit. This article presents selected examples of neurotrophic factors that are currently being tested in animal models or developed for transfer to the clinic, with a major focus on factors with the potential of becoming applicable in various forms of retinal degeneration.

Regulation and effects of GDF-15 in the retina following optic nerve crush.

Growth/differentiation factor-15 (GDF-15) is a distant member of the transforming growth factor-ß superfamily and is ubiquitously expressed in the central nervous system. It is prominently upregulated in cerebral cortical and ischemic lesion paradigms. GDF-15 robustly promotes the survival of lesioned nigrostriatal dopaminergic neurons in vivo; GDF-15-deficient mice exhibit progressive postnatal motor and sensory neuron losses implying essential functions of GDF-15 in neuronal survival. We show that GDF-15 mRNA and protein are, respectively, six-fold and three-fold upregulated in the murine retina at 1 day after optic nerve crush, slightly elevated mRNA levels being maintained until day 28. However, the magnitude and time course of retinal ganglion cell (RGC) death are indistinguishable in knockout and control mice. Selected mRNAs implicated in the regulation of the death vs. survival of RGCs, including ATF3, Bad, Bcl-2 and caspase-8, were similarly regulated in both knockout and control retinae. Immunohistochemistry for tyrosine hydroxylase and choline acetyltransferase revealed no differences in staining patterns in the two genotypes. mRNA and protein levels of galanin, a putative neuroprotective factor and positive regulator of neuron survival and axonal regeneration, were prominently upregulated after crush in knockout retinae at day 3, as compared with control retinae, suggesting that GDF-15 acts as a physiological regulator of galanin. GDF-15 is therefore prominently upregulated in the retina after optic nerve crush but does not directly interfere with the magnitude and temporal progression of RGC death.

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.

Local substitution of GDF-15 improves axonal and sensory recovery after peripheral nerve injury.

The growth/differentiation factor-15, GDF-15, has been found to be secreted by Schwann cells in the lesioned peripheral nervous system. To investigate whether GDF-15 plays a role in peripheral nerve regeneration, we substituted exogenous GDF-15 into 10-mm sciatic nerve gaps in adult rats and compared functional and morphological regeneration to a vehicle control group. Over a period of 11 weeks, multiple functional assessments, including evaluation of pinch reflexes, the Static Sciatic Index and of electrophysiological parameters, were performed. Regenerated nerves were then morphometrically analyzed for the number and quality of regenerated myelinated axons. Substitution of GDF-15 significantly accelerated sensory recovery while the effects on motor recovery were less strong. Although the number of regenerated myelinated axons was significantly reduced after GDF-15 treatment, the regenerated axons displayed advanced maturation corroborating the results of the functional assessments. Our results suggest that GDF-15 is involved in the complex orchestration of peripheral nerve regeneration after lesion.

Fibroblast growth factor 2 regulates astrocyte differentiation in a region-specific manner in the hindbrain.

Fibroblast growth factor (FGF)-2 is known to have important pleiotropic effects in neuronal and glial cells during various physiological and pathological events. To investigate the role of endogenous FGF-2 in the differentiation of astrocytes, we studied the expression of glial fibrillary acidic protein (GFAP) in the hindbrain of the FGF-2 null mouse. GFAP was drastically decreased in a region-specific manner in the hindbrain of the adult and developing FGF-2 null mouse without an associated change in the expression of alternate markers for astrocytes. The deficit was evident in the astrocytes of pontine and medullary gray matter but not in the white matter. The astrocytes of the gray and white matter were seen to express FGF-2 and FGF receptors in a distinct pattern. The methylation of histone H3 at lysine 4 residue (H3K4me2) associated with the STAT (signal transducer and activator of transcription)-binding site of the GFAP promoter was significantly decreased in the gray matter of the FGF-2 null mouse, suggesting a role for FGF-2 in the epigenetic regulation of astrocyte differentiation in vivo. These observations underscore the importance of FGF-2 in astroglial differentiation in the hindbrain and the heterogeneity of astrocytes in their requirement for FGF-2 as a differentiation inducing signal.

Fibroblast growth factor-2 deficiency causes defects in adult hippocampal neurogenesis, which are not rescued by exogenous fibroblast growth factor-2.

Neurogenesis within the adult brain is restricted to selected areas, one of which is the dentate gyrus (DG). Several growth factors have been reported to affect neurogenesis in the adult DG. However, a role of fibroblast growth factor-2 (FGF-2) in adult hippocampal neurogenesis has not been firmly established. We have analyzed neurogenesis in the DG using in vivo and in vitro approaches. FGF-2(-/-) mice revealed no alterations in the number of proliferating cells but a significant decrease in the numbers of newly generated neurons. Moreover, FGF-2 added to hippocampal slice cultures from FGF-2(-/-) mice was unable to rescue the phenotype. Although an increase in death of neurogenic cells in the FGF-2-deficient DG could not be specifically demonstrated, there was a massive increase in global cell death in FGF-2(-/-) hippocampal slice cultures compared with slices from wild-type mice. Cell death could not be prevented by addition of FGF-2. Neutralization of endogenous FGF-2 in hippocampal slices did not interfere with neurogenesis in a short-term paradigm. Together, our data suggest that FGF-2 is essentially required for maturation of new neurons in adult hippocampal neurogenesis but is likely to operate synergistically in combination with other mechanisms/growth factors.

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.

Spontaneous bursting activity in the developing entorhinal cortex.

Periodic spontaneous activity represents an important attribute of the developing nervous system. The entorhinal cortex (EC) is a crucial component of the medial temporal lobe memory system. Yet, little is known about spontaneous activity in the immature EC. Here, we investigated spontaneous field potential (fp) activity and intrinsic firing patterns of medial EC layer III principal neurons in brain slices obtained from rats at the first two postnatal weeks. A fraction of immature layer III neurons spontaneously generated prolonged (2-20 s) voltage-dependent intrinsic bursting activity. Prolonged bursts were dependent on the extracellular concentration of Ca(2+) ([Ca(2+)](o)). Thus, reduction of [Ca(2+)](o) increased the fraction of neurons with prolonged bursting by inducing intrinsic bursts in regularly firing neurons. In 1 mm [Ca(2+)](o), the percentages of neurons showing prolonged bursts were 53%, 81%, and 29% at postnatal day 5 (P5)-P7, P8-P10, and P11-P13, respectively. Prolonged intrinsic bursting activity was blocked by buffering intracellular Ca(2+) with BAPTA, and by Cd(2+), flufenamic acid (FFA), or TTX, and was suppressed by nifedipine and riluzole, suggesting that the Ca(2+)-sensitive nonspecific cationic current (I(CAN)) and the persistent Na(+) current (I(Nap)) underlie this effect. Indeed, a 0.2-1 s suprathreshold current step stimulus elicited a terminated plateau potential in these neurons. fp recordings at P5-P7 showed periodic spontaneous glutamate receptor-mediated events (sharp fp events or prolonged fp bursts) which were blocked by FFA. Slow-wave network oscillations become a dominant pattern at P11-P13. We conclude that prolonged intrinsic bursting activity is a characteristic feature of developing medial EC layer III neurons that might be involved in neuronal and network maturation.

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.