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

Transforming growth factor β (TGF-β) pathway in the immunopathogenesis of multiple sclerosis (MS); molecular approaches

INTRODUCTION

Multiple sclerosis (MS) is an acute demyelinating disease with an autoimmune nature, followed by gradual neurodegeneration and enervating scar formation. Dysregulated immune response is a crucial dilemma contributing to the pathogenesis of MS. The role of chemokines and cytokines, such as transforming growth factor-β (TGF-β), have been recently highlighted regarding their altered expressions in MS. TGF-β has three isoforms, TGF-β1, TGF-β2, and TGF-β3, that are structurally similar; however, they can show different functions.

RESULTS

All three isoforms are known to induce immune tolerance by modifying Foxp3⁺ regulatory T cells. Nevertheless, there are controversial reports concerning the role of TGF-β1 and 2 in the progression of scar formation in MS. At the same time, these proteins also improve oligodendrocyte differentiation and have shown neuroprotective behavior, two cellular processes that suppress the pathogenesis of MS. TGF-β3 shares the same properties but is less likely contributes to scar formation, and its direct role in MS remains elusive.

DISCUSSION

To develop novel neuroimmunological treatment strategies for MS, the optimal strategy could be the one that causes immune modulation, induces neurogenesis, stimulates remyelination, and prevents excessive scar formation. Therefore, regarding its immunological properties, TGF-β could be an appropriate candidate; however, contradictory results of previous studies have questioned its role and therapeutic potential in MS. In this review article, we provide an overview of the role of TGF-β in immunopathogenesis of MS, related clinical and animal studies, and the treatment potential of TGF-β in MS, emphasizing the role of different TGF-β isoforms.

Role of Transforming Growth Factor Beta in Peripheral Nerve Regeneration: Cellular and Molecular Mechanisms

Peripheral nerve injury (PNI) is one of the most common concerns in trauma patients. Despite significant advances in repair surgeries, the outcome can still be unsatisfactory, resulting in morbidities such as loss of sensory or motor function and reduced quality of life. This highlights the need for more supportive strategies for nerve regrowth and adequate recovery. Multifunctional cytokine transforming growth factor-β (TGF-β) is essential for the development of the nervous system and is known for its neuroprotective functions. Accumulating evidence indicates its involvement in multiple cellular and molecular responses that are critical to peripheral nerve repair. Following PNI, TGF-β is released at the site of injury where it can initiate a series of phenotypic changes in Schwann cells (SCs), modulate immune cells, activate neuronal intrinsic growth capacity, and regulate blood nerve barrier (BNB) permeability, thus enhancing the regeneration of the nerves. Notably, TGF-β has already been applied experimentally in the treatment of PNI. These treatments with encouraging outcomes further demonstrate its regeneration-promoting capacity. Herein, we review the possible roles of TGF-β in peripheral nerve regeneration and discuss the underlying mechanisms, thus providing new cues for better treatment of PNI.

Glial-derived neurotrophic factor in human airway smooth muscle

Airway smooth muscle (ASM) cells modulate the local airway milieu via production of inflammatory mediators and growth factors including classical neurotrophins, such as brain-derived neurotrophic factor (BDNF). The glial cell-derived neurotrophic factor (GDNF) family of ligands (GFLs) are nonclassical neurotrophins and their role in the airway is barely understood. The major GFLs, GDNF and Neurturin (NRTN) bind to GDNF family receptor (GFR) α1 and α2 respectively that pair with Ret receptor to accomplish signaling. In this study, we found GDNF is expressed in human lung and increased in adult asthma, while human ASM expresses GDNF and its receptors. Accordingly, we used human ASM cells to test the hypothesis that ASM expression and autocrine signaling by GFLs regulate [Ca²⁺ ]_i . Serum-deprived ASM cells from non-asthmatics were exposed to 10 ng/ml GDNF or NRTN for 15 min (acute) or 24 h (chronic). In fura-2 loaded cells, acute GDNF or NRTN alone induced [Ca²⁺ ]_i responses, and further enhanced responses to 1 µM ACh or 10 µM histamine. Ret inhibitor (SPP86; 10 µM) or specific GDNF chelator GFRα1-Fc (1 µg/ml) showed roles of these receptors in GDNF effects. In contrast, NRTN did not enhance [Ca²⁺ ]_i response to histamine. Furthermore, conditioned media of nonasthmatic and asthmatic ASM cells showed GDNF secretion. SPP86, Ret inhibitor and GFRα1-Fc chelator markedly decreased [Ca²⁺ ]_i response compared with vehicle, highlighting autocrine effects of secreted GDNF. Chronic GDNF treatment increased histamine-induced myosin light chain phosphorylation. These novel data demonstrate GFLs particularly GDNF/GFRα1 influence ASM [Ca²⁺ ]_i and raise the possibility that GFLs are potential targets of airway hyperresponsiveness.

Growth Factors as Axon Guidance Molecules: Lessons From in vitro Studies

Growth cones at the tips of extending axons navigate through developing organisms by probing extracellular cues, which guide them through intermediate steps and onto final synaptic target sites. Widespread focus on a few guidance cue families has historically overshadowed potentially crucial roles of less well-studied growth factors in axon guidance. In fact, recent evidence suggests that a variety of growth factors have the ability to guide axons, affecting the targeting and morphogenesis of growth cones in vitro. This review summarizes in vitro experiments identifying responses and signaling mechanisms underlying axon morphogenesis caused by underappreciated growth factors.

Protective Functions of Reactive Astrocytes Following Central Nervous System Insult

Astrocytes play important roles in numerous central nervous system disorders including autoimmune inflammatory, hypoxic, and degenerative diseases such as Multiple Sclerosis, ischemic stroke, and Alzheimer’s disease. Depending on the spatial and temporal context, activated astrocytes may contribute to the pathogenesis, progression, and recovery of disease. Recent progress in the dissection of transcriptional responses to varying forms of central nervous system insult has shed light on the mechanisms that govern the complexity of reactive astrocyte functions. While a large body of research focuses on the pathogenic effects of reactive astrocytes, little is known about how they limit inflammation and contribute to tissue regeneration. However, these protective astrocyte pathways might be of relevance for the understanding of the underlying pathology in disease and may lead to novel targeted approaches to treat autoimmune inflammatory and degenerative disorders of the central nervous system. In this review article, we have revisited the emerging concept of protective astrocyte functions and discuss their role in the recovery from inflammatory and ischemic disease as well as their role in degenerative disorders. Focusing on soluble astrocyte derived mediators, we aggregate the existing knowledge on astrocyte functions in the maintenance of homeostasis as well as their reparative and tissue-protective function after acute lesions and in neurodegenerative disorders. Finally, we give an outlook of how these mediators may guide future therapeutic strategies to tackle yet untreatable disorders of the central nervous system.

On the Right Track to Treat Movement Disorders: Promising Therapeutic Approaches for Parkinson's and Huntington's Disease

Movement disorders are neurological conditions in which patients manifest a diverse range of movement impairments. Distinct structures within the basal ganglia of the brain, an area involved in movement regulation, are differentially affected for every disease. Among the most studied movement disorder conditions are Parkinson's (PD) and Huntington's disease (HD), in which the deregulation of the movement circuitry due to the loss of specific neuronal populations in basal ganglia is the underlying cause of motor symptoms. These symptoms are due to the loss principally of dopaminergic neurons of the substantia nigra (SN) par compacta and the GABAergic neurons of the striatum in PD and HD, respectively. Although these diseases were described in the 19th century, no effective treatment can slow down, reverse, or stop disease progression. Available pharmacological therapies have been focused on preventing or alleviating motor symptoms to improve the quality of life of patients, but these drugs are not able to mitigate the progressive neurodegeneration. Currently, considerable therapeutic advances have been achieved seeking a more efficacious and durable therapeutic effect. Here, we will focus on the new advances of several therapeutic approaches for PD and HD, starting with the available pharmacological treatments to alleviate the motor symptoms in both diseases. Then, we describe therapeutic strategies that aim to restore specific neuronal populations or their activity. Among the discussed strategies, the use of Neurotrophic factors (NTFs) and genetic approaches to prevent the neuronal loss in these diseases will be described. We will highlight strategies that have been evaluated in both Parkinson's and Huntington's patients, and also the ones with strong preclinical evidence. These current therapeutic techniques represent the most promising tools for the safe treatment of both diseases, specifically those aimed to avoid neuronal loss during disease progression.

Plasma membrane localization of the GFL receptor components: a nexus for receptor crosstalk

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) comprise a group of four homologous and potent growth factors that includes GDNF, neurturin (NRTN), artemin (ARTN), and persephin (PSPN). The survival, growth, and mitotic activities of the GFLs are conveyed by a single receptor tyrosine kinase, Ret. The GFLs do not bind directly to Ret in order to activate it, and instead bind with high affinity to glycerophosphatidylinositol (GPI)-anchored coreceptors called the GDNF family receptor-αs (GFRαs). Several mechanisms have recently been identified that influence the trafficking of Ret and GFRαs in and out of the plasma membrane, thereby affecting their availability for ligand binding, as well as their levels by targeting to degradative pathways. This review describes these mechanisms and their powerful effects on GFL signaling and function. We also describe the recent discovery that p75 and Ret form a signaling complex, also regulated by plasma membrane shuttling, that either enhances GFL survival signals or p75 pro-apoptotic signals, dependent on the cellular context.

Neurotrophins and glial cell line-derived neurotrophic factor in the ovary: physiological and pathophysiological implications

BACKGROUND

Neurotrophins [nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4)] and glial cell line-derived neurotrophic factor (GDNF) are soluble polypeptide growth factors that are widely recognized for their roles in promoting cell growth, survival and differentiation in several classes of neurons. Outside the nervous system, neurotrophin (NT) and GDNF signaling events have substantial roles in various non-neural tissues, including the ovary.

OBJECTIVE AND RATIONALE

The molecular mechanisms that promote and regulate follicular development and oocyte maturation have been extensively investigated. However, most information has been obtained from animal models. Even though the fundamental process is highly similar across species, the paracrine regulation of ovarian function in humans remains poorly characterized. Therefore, this review aims to summarize the expression and functional roles of NTs and GDNF in human ovarian biology and disorders, and to describe and propose the development of novel strategies for diagnosing, treating and preventing related abnormalities.

SEARCH METHODS

Relevant literature in the English language from 1990 to 2018 describing the role of NTs and GDNF in mammalian ovarian biology and phenotypes was comprehensively selected using PubMed, MEDLINE and Google Scholar.

OUTCOMES

Studies have shown that the neurotrophins NGF, BDNF, NT-3 and NT-4 as well as GDNF and their functional receptors are expressed in the human ovary. Recently, gathered experimental data suggest putative roles for NT and GDNF signaling in the direct control of ovarian function, including follicle assembly, activation of the primordial follicles, follicular growth and development, oocyte maturation, steroidogenesis, ovulation and corpus luteum formation. Additionally, crosstalk occurs between these ovarian regulators and the endocrine signaling system. Dysregulation of the NT system may negatively affect ovarian function, leading to reproductive pathology (decreased ovarian reserve, polycystic ovary syndrome and endometriosis), female infertility and even epithelial ovarian cancers.

WIDER IMPLICATIONS

A comprehensive understanding of the expression, actions and underlying molecular mechanisms of the NT/GDNF system in the human ovary is essential for novel approaches to therapeutic and diagnostic interventions in ovarian diseases and to develop more safe, effective methods of inducing ovulation in ART in the treatment of female infertility.

TGF-β1 Increases GDNF Production by Upregulating the Expression of GDNF and Furin in Human Granulosa-Lutein Cells

Glial cell line-derived neurotrophic factor (GDNF) is expressed at a high level in the human ovary and GDNF signaling is involved in the direct control of follicular activation and oocyte maturation. Transforming growth factor-β1 (TGF-β1) plays an important role in the regulation of various ovarian functions. Furin is an intracellular serine endopeptidase of the subtilisin family that is closely associated with the activation of multiple protein precursors. Despite the important roles of GDNF and TGF-β1 in the regulation of follicular development, whether TGF-β is able to regulate the expression and production of GDNF in human granulosa cells remains to be determined. The aim of this study was to investigate the effect of TGF-β1 on the production of GDNF and its underlying mechanisms in human granulosa-lutein (hGL) cells. We used two types of hGL cells (primary hGL cells and an established immortalized hGL cell line, SVOG cells) as study models. Our results show that TGF-β1 significantly induced the expression of GDNF and furin, which, in turn, increased the production of mature GDNF. Using a dual inhibition approach combining RNA interference and kinase inhibitors against cell signaling components, we showed that the TβRII type II receptor and ALK5 type I receptor are the principal receptors that mediated TGF-β1-induced cellular activity in hGL cells. Additionally, Sma- and Mad-related protein (SMAD)3 and SMAD4 are the downstream signaling transducers that mediate the biological response induced by TGF-β1. Furthermore, furin is the main proprotein convertase that induces the production of GDNF. These findings provide additional regulatory mechanisms by which an intrafollicular factor influences the production of another growth factor through a paracrine or autocrine interaction in hGL cells.

Neurotrophic factors hold promise for the future of Parkinson's disease treatment: is there a light at the end of the tunnel?

Parkinson's disease (PD) is the second most common neurodegenerative disorder and is characterized by a spectrum of clinicopathologic signs and a complex etiology. PD results from the degeneration of dopaminergic (DAergic) neurons in the substantia nigra. Current therapies for PD are only able to alleviate symptoms without stopping disease progression. In addition, the available therapeutic strategies do not have long-lasting effects. Furthermore, these therapies cause different ranges of adverse side effects. There is great interest in neurotrophic factors (NTFs) due to their ability to promote the survival of different neural cells. These factors are divided into four families: neurotrophins, neurokines, the glial cell line-derived NTF family of ligands, and the newly recognized cerebral DA NTF/mesencephalic astrocyte-derived NTF family. The protective and therapeutic effects of these factors on DAergic neurons make them suitable for the prevention of progressive cell loss in PD. Based on the above premise, we focus on the protective effects of NTFs, especially CDNF and MANF, on nigrostriatal DAergic neurons in PD.

The GDNF Family: A Role in Cancer?

The glial cell line-derived neurotrophic factor (GDNF) family of ligands (GFLs) comprising of GDNF, neurturin, artemin, and persephin plays an important role in the development and maintenance of the central and peripheral nervous system, renal morphogenesis, and spermatogenesis. Here we review our current understanding of GFL biology, and supported by recent progress in the area, we examine their emerging role in endocrine-related and other non-hormone-dependent solid neoplasms. The ability of GFLs to elicit actions that resemble those perturbed in an oncogenic phenotype, alongside mounting evidence of GFL involvement in tumor progression, presents novel opportunities for therapeutic intervention.

TGF-β Family Signaling in Neural and Neuronal Differentiation, Development, and Function

Signaling by the transforming growth factor β (TGF-β) family is necessary for proper neural development and function throughout life. Sequential waves of activation, inhibition, and reactivation of TGF-β family members regulate numerous elements of the nervous system from the earliest stages of embryogenesis through adulthood. This review discusses the expression, regulation, and function of TGF-β family members in the central nervous system at various developmental stages, beginning with induction and patterning of the nervous system to their importance in the adult as modulators of inflammatory response and involvement in degenerative diseases.

Evidence for an Additive Neurorestorative Effect of Simultaneously Administered CDNF and GDNF in Hemiparkinsonian Rats: Implications for Different Mechanism of Action

Parkinson's disease (PD) is a neurodegenerative disorder associated with a progressive loss of dopaminergic (DAergic) neurons of the substantia nigra (SN) and the accumulation of intracellular inclusions containing α-synuclein. Current therapies do not stop the progression of the disease, and the efficacy of these treatments wanes over time. Neurotrophic factors (NTFs) are naturally occurring proteins promoting the survival and differentiation of neurons and the maintenance of neuronal contacts. CDNF (cerebral dopamine NTF) and GDNF (glial cell line-derived NTF) are able to protect DAergic neurons against toxin-induced degeneration in experimental models of PD. Here, we report an additive neurorestorative effect of coadministration of CDNF and GDNF in the unilateral 6-hydroxydopamine (6-OHDA) lesion model of PD in rats. NTFs were given into the striatum four weeks after unilateral intrastriatal injection of 6-OHDA (20 µg). Amphetamine-induced (2.5 mg/kg, i.p.) rotational behavior was measured every two weeks. Number of tyrosine hydroxylase (TH)-positive cells from SN pars compacta (SNpc) and density of TH-positive fibers in the striatum were analyzed at 12 weeks after lesion. CDNF and GDNF alone restored the DAergic function, and one specific dose combination had an additive effect: CDNF (2.5µg) and GDNF (1µg) coadministration led to a stronger trophic effect relative to either of the single treatments alone. The additive effect may indicate different mechanism of action for the NTFs. Indeed, both NTFs activated the survival promoting PI3 kinase (PI3K)-Akt signaling pathway, but only CDNF decreased the expression level of tested endoplasmatic reticulum (ER) stress markers ATF6, glucose-regulated protein 78 (GRP78), and phosphorylation of eukaryotic initiation factor 2α subunit (eIF2α).

Current disease modifying approaches to treat Parkinson's disease

Parkinson's disease (PD is a progressive neurological disorder characterized by the degeneration and death of midbrain dopamine and non-dopamine neurons in the brain leading to motor dysfunctions and other symptoms, which seriously influence the quality of life of PD patients. The drug L-dopa can alleviate the motor symptoms in PD, but so far there are no rational therapies targeting the underlying neurodegenerative processes. Despite intensive research, the molecular mechanisms causing neuronal loss are not fully understood which has hampered the development of new drugs and disease-modifying therapies. Neurotrophic factors are by virtue of their survival promoting activities attract candidates to counteract and possibly halt cell degeneration in PD. In particular, studies employing glial cell line-derived neurotrophic factor (GDNF) and its family member neurturin (NRTN), as well as the recently described cerebral dopamine neurotrophic factor (CDNF) and the mesencephalic astrocyte-derived neurotrophic factor (MANF) have shown positive results in protecting and repairing dopaminergic neurons in various models of PD. Other substances with trophic actions in dopaminergic neurons include neuropeptides and small compounds that target different pathways impaired in PD, such as increased cell stress, protein handling defects, dysfunctional mitochondria and neuroinflammation. In this review, we will highlight the recent developments in this field with a focus on trophic factors and substances having the potential to beneficially influence the viability and functions of dopaminergic neurons as shown in preclinical or in animal models of PD.

Glial cell line-derived neurotrophic factor-mediated enhancement of noradrenergic descending inhibition in the locus coeruleus exerts prolonged analgesia in neuropathic pain

BACKGROUND AND PURPOSE

The locus coeruleus (LC) is the principal nucleus containing the noradrenergic neurons and is a major endogenous source of pain modulation in the brain. Glial cell line-derived neurotrophic factor (GDNF), a well-established neurotrophic factor for noradrenergic neurons, is a major pain modulator in the spinal cord and primary sensory neurons. However, it is unknown whether GDNF is involved in pain modulation in the LC.

EXPERIMENTAL APPROACH

Rats with chronic constriction injury (CCI) of the left sciatic nerve were used as a model of neuropathic pain. GDNF was injected into the left LC of rats with CCI for 3 consecutive days and changes in mechanical allodynia and thermal hyperalgesia were assessed. The α2 -adrenoceptor antagonist yohimbine was injected intrathecally to assess the involvement of descending inhibition in GDNF-mediated analgesia. The MEK inhibitor U0126 was used to investigate whether the ERK signalling pathway plays a role in the analgesic effects of GDNF.

KEY RESULTS

Both mechanical allodynia and thermal hyperalgesia were attenuated 24 h after the first GDNF injection. GDNF increased the noradrenaline content in the dorsal spinal cord. The analgesic effects continued for at least 3 days after the last injection. Yohimbine abolished these effects of GDNF. The analgesic effects of GDNF were partly, but significantly, inhibited by prior injection of U0126 into the LC.

CONCLUSIONS AND IMPLICATIONS

GDNF injection into the LC exerts prolonged analgesic effects on neuropathic pain in rats by enhancing descending noradrenergic inhibition.

How do Chinese medicines that tonify the kidney inhibit dopaminergic neuron apoptosis?

Wistar rats were intragastrically perfused with Chinese medicines used for tonifying the kidney. These included 0.180 g/mL of Herba Epimedii (Epimedium), Semen Cuscutae (Dodder Seed), or Herba Cistanches (Desertliving Cistanche), 0.04 mg/mL monoamine oxidase-B inhibitor selegiline, or distilled water for 14 consecutive days to prepare drug-containing serum or blank serum. MES23.5 cells in the logarithmic phase were cultured in media supplemented with 15% drug-containing serum for 24 hours, followed by incubation in culture solution containing 100 μmol/L H2O2 for 3 hours. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and flow tometry results showed that all drug-containing serums improved the survival rate of H2O2-injured MES23.5 cells, inhibited pro-apoptotic FasL and caspase-3 expression, promoted anti-apoptotic Bcl-2 expression. However, drug-containing serums had little influence on Fas expression in H2O2-injured MES23.5 cells. Enzyme-linked immunosorbent assay results showed that serum containing Herba Cistanches or Herba Epimedii increased the expression of nerve growth factor, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor in injured MES23.5 cells; serum containing Semen Cuscutae only increased brain-derived neurotrophic factor expression; while expression of the above neurotrophic factors remained the same in cells treated with serum containing selegiline. These findings indicate that Chinese medicines used to tonify the kidney can protect nerve cells by regulating the expression of apoptosis-related factors and neuro-trophic factors in MES23.5 cells.

Concerted interaction of TGF-β and GDNF mediates neuronal differentiation

The glial cell-line derived neurotrophic factor (GDNF) is crucial for ureteric bud morphogenesis, spermatogenesis, and development of the enteric nervous system and is a potent survival factor for various neuronal populations. However, the impact of GDNF, at least on cell survival, was found to depend strongly on the presence of transforming growth factor β (TGF-β). In this study, we investigate the role of TGF-β in GDNF-induced neuronal differentiation. In a cell culture paradigm of N2aGT cells (neuroblastoma cell line), we show that TGF-β signaling localizes the GDNF ligand-binding receptor GFRa1 to the cell surface, which is a known mechanism by which TGF-β is able to facilitate GDNF signaling. TGF-β-mediated GDNF signaling slightly elevated the phosphorylation state of Ret, the canonical coreceptor for the GPI-linked (glycosyl-phosphatidylinositol) GFRa1. On the basis of morphological as well as immunocytological data, we finally show that GDNF-mediated neuronal differentiation is intensified when GDNF and TGF-β act in concert.

Roles for the TGFβ superfamily in the development and survival of midbrain dopaminergic neurons

The adult midbrain contains 75% of all dopaminergic neurons in the CNS. Within the midbrain, these neurons are divided into three anatomically and functionally distinct clusters termed A8, A9 and A10. The A9 group plays a functionally non-redundant role in the control of voluntary movement, which is highlighted by the motor syndrome that results from their progressive degeneration in the neurodegenerative disorder, Parkinson's disease. Despite 50 years of investigation, treatment for Parkinson's disease remains symptomatic, but an intensive research effort has proposed delivering neurotrophic factors to the brain to protect the remaining dopaminergic neurons, or using these neurotrophic factors to differentiate dopaminergic neurons from stem cell sources for cell transplantation. Most neurotrophic factors studied in this context have been members of the transforming growth factor β (TGFβ) superfamily. In recent years, an intensive research effort has focused on understanding the function of these proteins in midbrain dopaminergic neuron development and their role in the molecular architecture that regulates the development of this brain region, with the goal of applying this knowledge to develop novel therapies for Parkinson's disease. In this review, the current evidence showing that TGFβ superfamily members play critical roles in the regulation of midbrain dopaminergic neuron induction, differentiation, target innervation and survival during embryonic and postnatal development is analysed, and the implications of these findings are discussed.

TGF-β signaling regulates neuronal C1q expression and developmental synaptic refinement

Immune molecules, including complement proteins C1q and C3, have emerged as critical mediators of synaptic refinement and plasticity. Complement localizes to synapses and refines the developing visual system via C3-dependent microglial phagocytosis of synapses. Retinal ganglion cells (RGCs) express C1q, the initiating protein of the classical complement cascade, during retinogeniculate refinement; however, the signals controlling C1q expression and function remain elusive. Previous work implicated an astrocyte-derived factor in regulating neuronal C1q expression. Here we identify retinal TGF-β as a key regulator of neuronal C1q expression and synaptic pruning in the developing visual system. Mice lacking TGF-β receptor II (TGFβRII) in retinal neurons have reduced C1q expression in RGCs, reduced synaptic localization of complement, and phenocopy refinement defects observed in complement-deficient mice, including reduced eye specific segregation and microglial engulfment of RGC inputs. These data implicate TGF-β in regulating neuronal C1q expression to initiate complement- and microglia-mediated synaptic pruning.

Restoration versus reconstruction: cellular mechanisms of skin, nerve and muscle regeneration compared

In tissues characterized by a high turnover or following acute injury, regeneration replaces damaged cells and is involved in adaptation to external cues, leading to homeostasis of many tissues during adult life. An understanding of the mechanics underlying tissue regeneration is highly relevant to regenerative medicine-based interventions. In order to investigate the existence a leitmotif of tissue regeneration, we compared the cellular aspects of regeneration of skin, nerve and skeletal muscle, three organs characterized by different types of anatomical and functional organization. Epidermis is a stratified squamous epithelium that migrates from the edge of the wound on the underlying dermis to rebuild lost tissue. Peripheral neurons are elongated cells whose neurites are organized in bundles, within an endoneurium of connective tissue; they either die upon injury or undergo remodeling and axon regrowth. Skeletal muscle is characterized by elongated syncytial cells, i.e. muscle fibers, that can temporarily survive in broken pieces; satellite cells residing along the fibers form new fibers, which ultimately fuse with the old ones as well as with each other to restore the previous organization. Satellite cell asymmetrical division grants a reservoir of undifferentiated cells, while other stem cell populations of muscle and non-muscle origin participate in muscle renewal. Following damage, all the tissues analyzed here go through three phases: inflammation, regeneration and maturation. Another common feature is the occurrence of cellular de-differentiation and/or differentiation events, including gene transcription, which are typical of embryonic development. Nonetheless, various strategies are used by different tissues to replace their lost parts. The epidermis regenerates ex novo, whereas neurons restore their missing parts; muscle fibers use a mixed strategy, based on the regrowth of missing parts through reconstruction by means of newborn fibers. The choice of either strategy is influenced by the anatomical, physical and chemical features of the cells as well as by the extracellular matrix typical of a given tissue, which points to the existence of differential, evolutionary-based mechanisms for specific tissue regeneration. The shared, ordered sequence of steps that characterize the regeneration processes examined suggests it may be possible to model this extremely important phenomenon to reproduce multicellular organisms.

GDNF, NGF and BDNF as therapeutic options for neurodegeneration

Glial cell-derived neurotrophic factor (GDNF), and the neurotrophin nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are important for the survival, maintenance and regeneration of specific neuronal populations in the adult brain. Depletion of these neurotrophic factors has been linked with disease pathology and symptoms, and replacement strategies are considered as potential therapeutics for neurodegenerative diseases such as Parkinson's, Alzheimer's and Huntington's diseases. GDNF administration has recently been shown to be an effective treatment for Parkinson's disease, with clinical trials currently in progress. Trials with NGF for Alzheimer's disease are ongoing, with some degree of success. Preclinical results using BDNF also show much promise, although there are accompanying difficulties. Ultimately, the administration of a therapy involving proteins in the brain has inherent problems. Because of the blood-brain-barrier, the protein must be infused directly, produced by viral constructs, secreted from implanted protein-secreting cells or actively transported across the brain. An alternative to this is the use of a small molecule agonist, a modulator or enhancer targeting the associated receptors. We evaluate these neurotrophic factors as potential short or long-term treatments, weighing up preclinical and clinical results with the possible effects on the underlying neurodegenerative process.

Evidence for increased tissue androgen sensitivity in neurturin knockout mice

Neurturin (NTN) is a member of the glial cell line-derived neurotrophic factor (GDNF) family and signals through GDNF family receptor alpha 2 (GFRα2). We hypothesised that epithelial atrophy reported in the reproductive organs of Ntn (Nrtn)- and Gfrα2 (Gfra2)-deficient mice could be due to NTN affecting the hormonal environment. To investigate this, we compared the reproductive organs of Ntn- and Gfrα2-deficient male mice in parallel with an analysis of their circulating reproductive hormone levels. There were no significant structural changes within the organs of the knockout mice; however, serum and intratesticular testosterone and serum LH levels were very low. To reconcile these observations, we tested androgen sensitivity by creating a dihydrotestosterone (DHT) clamp (castration plus DHT implant) to create fixed circulating levels of androgens, allowing the evaluation of androgen-sensitive endpoints. At the same serum DHT levels, serum LH levels were lower and prostate and seminal vesicle weights were higher in the Ntn knockout (NTNKO) mice than in the wild-type mice, suggesting an increased response to androgens in the accessory glands and hypothalamus and pituitary of the NTNKO mice. Testicular and pituitary responsiveness was unaffected in the NTNKO males, as determined by the response to the human chorionic gonadotrophin or GNRH analogue, leuprolide, respectively. In conclusion, our results suggest that NTN inactivation enhances androgen sensitivity in reproductive and neuroendocrine tissues, revealing a novel mechanism to influence reproductive function and the activity of other androgen-dependent tissues.

Schwann cells migrate along axons in the absence of GDNF signaling

Background

During development neural crest derived Schwann Cell (SC) precursors migrate to nerve trunks and populate nascent nerves. Axonal ensheathment by SC is a prerequisite for normal nerve function and the integrity of myelinated as well as nonmyelinated axons. To provide adequate support functions, SC colonize entire nerves. One important prerequisite for this is their migration into distal axonal regions.

Results

Here, we studied the role of Glial cell line derived neurotrophic factor (GDNF), a TGF-beta related growth factor, for SC migration. To this end we used a superior cervical ganglion (SCG) explant-SC migration assay, GDNF null mutant mouse embryos and a chemical inhibitor for GDNF signaling in combination with time-lapse imaging. We found that GDNF signaling is dispensable for SC migration along murine embryonic sympathetic axons. Furthermore, in vivo analyzes revealed that SC migration along the sciatic nerve is also not dependent on GDNF.

Conclusions

In contrast to previous in vitro findings in the sciatic nerve and a SC precursor cell line, our results clearly indicate that GDNF is dispensable for embryonic SC migration. This is demonstrated for the sympathetic nervous system and also for the sciatic nerve in mouse.

Activation of JNK signaling mediates connective tissue growth factor expression and scar formation in corneal wound healing

Connective Tissue Growth Factor (CTGF) and Transforming growth factor-β1 (TGF-β1) are key growth factors in regulating corneal scarring. Although CTGF was induced by TGF-β1 and mediated many of fibroproliferative effects of TGF-β1, the signaling pathway for CTGF production in corneal scarring remains to be clarified. In the present study, we firstly investigated the effects of c-Jun N-terminal kinase (JNK) on CTGF expression induce by TGF-β1 in Telomerase-immortalized human cornea stroma fibroblasts (THSF). Then, we created penetrating corneal wound model and determined the effect of JNK in the pathogenesis of corneal scarring. TGF-β1 activated MAPK pathways in THSF cells. JNK inhibitor significantly inhibited CTGF, fibronectin and collagen I expression induced by TGF-β1 in THSF. In corneal wound healing, the JNK inhibitor significantly inhibited CTGF expression, markedly improved the architecture of corneal stroma and reduced corneal scar formation, but did not have a measurable impact on corneal wound healing in vivo. Our results indicate that JNK mediates the expression of CTGF and corneal scarring in corneal wound healing, and might be considered as specific targets of drug therapy for corneal scarring.

Glial cell-line derived neurotrophic factor is essential for electroconvulsive shock-induced neuroprotection in an animal model of Parkinson's disease

Sustained motor improvement in human patients with idiopathic Parkinson's disease has been described following electroconvulsive shock (ECS) treatment. In rats, ECS stimulates the expression of various trophic factors (TFs), some of which have been proposed to exert neuroprotective actions. We previously reported that ECS protects the integrity of the rat nigrostriatal dopaminergic system against 6-hydroxydopamine (6-OHDA)-induced toxicity; in order to shed light into its neuroprotective mechanism, we studied glial cell-line derived neurotrophic factor (GDNF) levels (the most efficient TF for dopaminergic neurons) in the substantia nigra (SN) and striatum of 6-OHDA-injected animals with or without ECS treatment. 6-OHDA injection decreased GDNF levels in the SN control animals, but not in those receiving chronic ECS, suggesting that changes in GDNF expression may participate in the ECS neuroprotective mechanism. To evaluate this possibility, we inhibit GDNF by infusion of GDNF function blocking antibodies in the SN of 6-OHDA-injected animals treated with ECS (or sham ECS). Animals were sacrificed 7 days after 6-OHDA infusion, and the integrity of the nigrostriatal system was studied by tyrosine hydroxylase immunohistochemistry and Cresyl Violet staining. Neuroprotection observed in ECS-treated animals was inhibited by GDNF antibodies in the SN. These results robustly demonstrate that GDNF is essential for the ECS neuroprotective effect observed in 6-OHDA-injected animals.

Effects of glial cell line-derived neurotrophic factor on dental pulp cells

This study investigated the effects of glial cell line-derived neurotrophic factor (GDNF) on dental pulp cells (DPCs). Cultures of DPCs expressed GDNF as well as its receptors, GFRα1 and RET. Addition of recombinant GDNF to cultures in serum-containing medium did not significantly affect DPC growth; however, GDNF dose-dependently increased viable cell number under serum-free culture conditions. Live/dead, lactate dehydrogenase (LDH), and caspases-3/-7 assays demonstrated that cell death occurred under serum-free conditions, and that GDNF significantly reduced the number of dead cells by inhibiting apoptotic cell death. GDNF also stimulated cell proliferation in serum-free conditions, as assessed by the BrdU incorporation assay. The effect of GDNF was abolished in the presence of inhibitors to GFRα1 and RET suggesting receptor-mediated events. This study also demonstrated that GDNF counteracted TNFα-induced DPC cytotoxicity, suggesting that GDNF may be cytoprotective under disease conditions. In conclusion, our findings indicate that GDNF promotes cell survival and proliferation of DPCs and suggest that GDNF may play a multifunctional role in the regulation of dental pulp homeostasis.

F-spondin regulates neuronal survival through activation of disabled-1 in the chicken ciliary ganglion

The extracellular membrane-associated protein F-spondin has been implicated in cell-matrix and cell-cell adhesion and plays an important role in axonal pathfinding. We report here that F-spondin is expressed in non-neuronal cells in the embryonic chicken ciliary ganglion (CG) and robustly promotes survival of cultured CG neurons. Using deletion constructs of F-spondin we found that the amino-terminal Reelin/Spondin domain cooperates with thrombospondin type 1 repeat (TSR) 6, a functional TGFβ-activation domain. In ovo treatment with blocking antibodies raised against the Reelin/Spondin domain or the TSR-domains caused increased apoptosis of CG neurons during the phase of programmed cell death and loss of about 30% of the neurons compared to controls. The Reelin/Spondin domain receptor - APP and its downstream signalling molecule disabled-1 are expressed in CG neurons. F-spondin induced rapid phosphorylation of disabled-1. Moreover, both blocking the central APP domain and interference with disabled-1 signalling disrupted the survival promoting effect of F-spondin. Taken together, our data suggest that F-spondin can promote neuron survival by a mechanism involving the Reelin/Spondin and the TSR domains.

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

GDF-15 is a novel distant member of the TGF-β 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.

Advances in understanding the role of neurotrophins in physiological and pathological processes in the intestinal tract

Neurotrophins (NT) play an important role in the process of neuronal growth, development, protection and repair. In recent years, great advances have been achieved in the research of neurotrophins. Glial cell line-derived neurotrophic factor is a nutritional factor that has the most potential to promote neuronal growth, differentiation and repair. It can also modulate intestinal neuronal growth and neurotransmitter expression. Brain-derived neurotrophic factor plays an important role in the development of the enteric nervous system, intestinal infection, and the modulation of gastrointestinal motility. Neurotrophin-3 can increase excitatory peptides that are expressed by neurons in the intestinal muscularis to accelerate colonic transit. Ciliary neurotrophic factor and other neurotrophins have a synergistic effect on neurons. In this article, we will review the recent advances in understanding the role of neurotrophins in physiological and pathological processes in the intestinal tract.

Aged Tgfbeta2/Gdnf double-heterozygous mice show no morphological and functional alterations in the nigrostriatal system

Loss of dopaminergic neurons in the substantia nigra pars compacta and the resulting decrease in striatal dopamine levels are the hallmarks of Parkinson’s disease. Tgfβ and Gdnf have been identified as neurotrophic factors for dopaminergic midbrain neurons in vivo and in vitro. Haploinsufficiency for either Tgfβ or Gdnf led to dopaminergic deficits. In this study we therefore analyzed the nigrostriatal system of aged Tgfβ2^+/−/Gdnf^+/− double-heterozygous mice. Unexpectedly, we found no morphological changes in the nigrostriatal system as compared with age-matched wild-type mice. There were no significant differences in the number of TH-positive midbrain neurons and no changes in the optical density of TH immunoreactivity in striata of Tgfβ2^+/−/Gdnf^+/− double-heterozygous mice. Moreover, we found no significant differences in the striatal levels of dopamine and its metabolites dihydroxyphenylacetic acid and homovanillic acid. Our results indicate that a combined haploinsufficiency for Tgfβ2 and Gdnf has no impact on the function and the survival of midbrain DA neurons under normal aging conditions.

Specificity of the second messenger pathways involved in basic fibroblast growth factor-induced survival and neurite growth in chick ciliary ganglion neurons

Basic fibroblast growth factor (bFGF) exerts multiple neurotrophic actions on cultured neurons from the ciliary ganglion of chick embryo, among them promotion of neuronal survival and of neurite outgrowth. To understand the specificity of the signal transduction cascades involved in the control of these processes, we used pharmacological inhibitors of the three main effectors known to act downstream of the bFGF receptor (FGFR): phospholipase Cgamma (PLCgamma), mitogen-activated protein kinase (MAPK), and phosphatidylinositol 3-kinase (PI3-K). Neuronal survival was assessed at 24 and 48 hr; neurite growth was analyzed both on dissociated neurons and on explants of whole ganglia. Our data show that only the PI3-K pathway is involved in the survival-promoting effect of bFGF; on the other hand, all three effectors converge on the enhancement of neurite outgrowth, both on isolated neurons and in whole ganglia.

Morphophysiology of the Zuckerkandl's paraganglion: effects of dexamethasone and aging

The extra-adrenal Zuckerkandl's paraganglion is used as a source of chromaffin cells for transplantation in parkinsonian animals. Aging can affect its viability, and this tissue needs further characterization for improving grafting procedures. The objectives were: (i) to compare the main morpho-functional characteristics of prepubertal and old Zuckerkandl's paraganglion (ZP), and (ii) to discern phenotypic changes after sub-chronic dexamethasone treatment in extra-adrenal tissue of prepubertal rats. For these purposes, immunostaining methods, stereology, voltammetry, cell culture, Western blotting, and ELISA were employed. The findings revealed that all paraganglia were composed of mesenchymal tissue and chromaffin cells. In prepubertal rats, chromaffin cells are arranged as large or small clusters. Large clusters (also known as "cell nests") contain densely packed chromaffin cells, and they are seen as fascicles in longitudinal sections. In old paraganglia, cell nests disappear, and chromaffin cells are found to be arranged as small cell clusters or dispersed throughout the mesenchyma. Paraganglionic chromaffin cells possess a rounded morphology with diameter ranging from 12 to 15 μm, with intracytoplasmic granules (100-500 nm in diameter) containing catecholamines. Prepubertal and old ZP chromaffin cells are mostly noradrenergics, and a few of them are dopaminergics. Aging reduces the amount of chromaffin tissue (28% in adult rats vs. 11% in old animals, both in relation to total volume of the paraganglion), and induces the presence of adrenergic cells and adrenaline. Both prepubertal and old cells express the neurotrophic factors GDNF and TGF-β₁, aging leading to reduced levels of both growth factors. Dexamethasone (50 μg/kg daily, 5 days) leads to the expression of phenylethanolamine-N-methyl-transferase in prepubertal paraganglia, and to a higher content and release of adrenaline.

Ischemic brain cell-derived conditioned medium protects astrocytes against ischemia through GDNF/ERK/NF-kB signaling pathway

Conditioned medium (CM) collected from cultures of ischemic microglia, astrocytes, and neurons were protective to astrocytes under the in vitro ischemic condition (deprivation of oxygen, glucose and serum). Molecular and signaling pathway(s) responsible for the CMs protective activity were investigated. Results showed that CMs from the ischemic microglia (MCM), astrocytes (ACM) and neurons (NCM) contained glial cell line-derived neurotrophic factor (GDNF), which protects astrocytes against the in vitro ischemia. Expression of extra cellular signal-regulated kinase (ERK1/2) and nuclear factor-kappa B (NF-kB) by GDNF led to the inhibition of apoptosis of the ischemic astrocytes in a caspase 3-independent manner. However, CMs- and GDNF-mediated protection of the ischemic astrocytes was protein kinase B (Akt) independent. These results provided mechanistic data regarding how GDNF- and CMs-mediated protection of the ischemic astrocytes is taking place. These observations provide information for the use of GDNF and GDNF containing CMs in the control of cerebral ischemia.

GDNF hyperalgesia is mediated by PLCgamma, MAPK/ERK, PI3K, CDK5 and Src family kinase signaling and dependent on the IB4-binding protein versican

The function of the isolectin B4 (IB4+)-binding and GDNF-dependent Ret (Ret+)-expressing non-peptidergic subpopulation of nociceptors remain poorly understood. We demonstrate that acute administration of GDNF sensitizes nociceptors and produces mechanical hyperalgesia in the rat. Intrathecal IB4-saporin, a selective toxin for IB4+/Ret+-nociceptors, attenuates GDNF but not NGF hyperalgesia. Conversely, intrathecal antisense to Trk A attenuated NGF but not GDNF hyperalgesia. Intrathecal administration of antisense oligodeoxynucleotides targeting mRNA for versican, the molecule that renders the Ret-expressing nociceptors IB4-positive (+), also attenuated GDNF but not NGF hyperalgesia, as did ADAMTS-4, a matrix metalloprotease known to degrade versican. Finally, inhibitors for all five signaling pathways known to be activated by GDNF at GFRa1/Ret: PLCc, CDK5, PI3K,MAPK/ERK and Src family kinases, attenuated GDNF hyperalgesia. Our results demonstrate a role of the non-peptidergic nociceptors in pain produced by the neurotrophin GDNF and suggest that the IB4-binding protein versican functions in the expression of this phenotype.

Transforming growth factor beta cooperates with persephin for dopaminergic phenotype induction

The aim of the present study was to investigate the putative cooperative effects of transforming growth factor beta (TGF-beta) and glial cell line-derived neurotrophic factor (GDNF) family ligands in the differentiation of midbrain progenitors toward a dopaminergic phenotype. Therefore, a mouse midbrain embryonic day (E) 12 neurospheres culture was used as an experimental model. We show that neurturin and persephin (PSPN), but not GDNF, are capable of transient induction of dopaminergic neurons in vitro. This process, however, requires the presence of endogenous TGF-beta. In contrast, after 8 days in vitro GDNF rescued the TGF-beta neutralization-dependent loss of the TH-positive cells. In vivo, at E14.5, no apparent phenotype concerning dopaminergic neurons was observed in Tgf-beta2(-/-)/gdnf(-/-) double mutant mice. In vitro, combined TGF-beta/PSPN treatment achieved a yield of approximately 20% TH-positive cells that were less vulnerable against 1-methyl-4-phenyl pyridinium ion toxicity. The underlying TGF-beta/PSPN differentiation signaling is receptor-mediated, involving p38 mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways. These results indicate that phenotype induction and survival of fully differentiated neurons are accomplished through distinct pathways and individual factor requirement. TGF-beta is required for the induction of dopaminergic neurons, whereas GDNF is required for regulating and/or maintaining a differentiated neuronal phenotype. Moreover, this study suggests that the combination of TGF-beta with PSPN is a potent inductive cocktail for the generation of dopaminergic neurons that should be considered in tissue engineering and cell replacement therapies for Parkinson's disease.

A role for glial cell derived neurotrophic factor induced expression by inflammatory cytokines and RET/GFR alpha 1 receptor up-regulation in breast cancer

By screening a tissue microarray of invasive breast tumors, we have shown that the receptor tyrosine kinase RET (REarranged during Transfection) and its coreceptor GFR alpha 1 (GDNF receptor family alpha-1) are overexpressed in a subset of estrogen receptor-positive tumors. Germ line-activating oncogenic mutations in RET allow this receptor to signal independently of GFR alpha 1 and its ligand glial cell-derived neurotrophic factor (GDNF) to promote a spectrum of endocrine neoplasias. However, it is not known whether tumor progression can also be driven by receptor overexpression and whether expression of GDNF, as has been suggested for other neurotrophic factors, is regulated in response to the inflammatory microenvironment surrounding many epithelial cancers. Here, we show that GDNF stimulation of RET(+)/GFR alpha 1(+) MCF7 breast cancer cells in vitro enhanced cell proliferation and survival, and promoted cell scattering. Moreover, in tumor xenografts, GDNF expression was found to be up-regulated on the infiltrating endogenous fibroblasts and to a lesser extent by the tumor cells themselves. Finally, the inflammatory cytokines tumor necrosis factor-alpha and interleukin-1 beta, which are involved in tumor promotion and development, were found to act synergistically to up-regulate GDNF expression in both fibroblasts and tumor cells. These data indicate that GDNF can act as an important component of the inflammatory response in breast cancers and that its effects are mediated by both paracrine and autocrine stimulation of tumor cells via signaling through the RET and GFR alpha 1 receptors.

RET signaling does not modulate MPTP toxicity but is required for regeneration of dopaminergic axon terminals

Activation of the RET (rearranged during transfection) receptor by glial cell-line-derived neurotrophic factor (GDNF) has been identified as an important differentiation and survival factor for dopaminergic neurons of the midbrain in preclinical experiments. These encouraging results have led to clinical trials of GDNF in patients with Parkinson's disease, which have resulted in conflicting findings. To investigate the potential benefit of Ret-dependent signaling on the challenged dopaminergic system, we tested the effect of tissue-selective ablation of the Ret gene on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity in mice, the most widely used animal model for Parkinson's disease. Ablation of Ret did not modify the MPTP-induced loss of dopaminergic neurons in the substantia nigra pars compacta and the dopaminergic innervation of the striatum at 14 days. However, Ret ablation abolished the regeneration of dopaminergic fibers and terminals, as well as the partial recovery of striatal dopamine concentrations, that was observed in control mice between days 14 and 90 after MPTP treatment. We therefore conclude that RET signaling has no influence on the survival of dopaminergic neurons in the MPTP model of Parkinson's disease but rather facilitates the regeneration of dopaminergic axon terminals.

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.

How to make a mesodiencephalic dopaminergic neuron

Dopaminergic neurons located in the ventral mesodiencephalon are essential for the control of voluntary movement and the regulation of emotion, and are severely affected in neurodegenerative diseases such as Parkinson's disease. Recent advances in molecular biology and mouse genetics have helped to unravel the mechanisms involved in the development of mesodiencephalic dopaminergic (mdDA) neurons, including their specification, migration and differentiation, as well as the processes that govern axonal pathfinding and their specific patterns of connectivity and maintenance. Here, we follow the developmental path of these neurons with the goal of generating a molecular code that could be exploited in cell-replacement strategies to treat diseases such as Parkinson's disease.

GDNF applied to the MPTP-lesioned nigrostriatal system requires TGF-beta for its neuroprotective action

GDNF is a potent neurotrophic factor for nigrostriatal dopaminergic neurons in vitro and in animal models of Parkinson's disease (PD), but has largely failed when tested in therapeutic applications in human PD. We report here that GDNF requires transforming growth factor-beta (TGF-beta) to elicit its neurotrophic activity. Lesioning the mouse nigrostriatal system with MPTP significantly upregulates striatal TGF-beta2 mRNA levels. As expected, GDNF protects against the destructive effects of MPTP, including losses of TH-ir nigral neurons, striatal dopamine and TH-ir fibers. Application of antibodies neutralizing all three TGF-beta isoforms to the MPTP-lesioned striatum abolishes the neurotrophic effect of GDNF. We show that TGF-beta antibodies are not toxic and do not interfere with retrograde transport of iodinated GDNF, suggesting that TGF-beta antibodies do not impair internalization and retrograde trafficking of GDNF. We conclude that striatal TGF-beta may be essential for permitting exogenous GDNF to act as a neuroprotective factor.

Differential brain-derived neurotrophic factor and transforming growth factor-beta expression in the rat cochlea following deafness

Brain derived neurotrophic factor (BDNF) and transforming growth factor-beta (TGFbeta) subtypes have demonstrated their importance in cochlear functions. The aim of this study was to determine gene and protein expression patterns of BDNF, TGFbeta1/2 and their main receptors trkB and TGFbetaR1/R2 in the auditory nerve and inferior colliculus of normal hearing and deafened rats by reverse-transcriptase polymerase chain reaction and immunohistochemistry. Deafening was performed by cochlear injection of neomycin (10%). Significant gene expression changes were not found in the inferior colliculus after deafness; however, in the auditory nerve, BDNF, TGFbeta1 and TGFbetaR1 mRNA in particular were upregulated. Additionally, BDNF protein and the cytokines were found to be expressed in the auditory nerve after hair cell loss. These data indicate the importance of BDNF and TGFbeta1 as endogenous survival factors.

Building Complex Brains – Missing Pieces in an Evolutionary Puzzle

The mechanisms underlying evolution of complex nervous systems are not well understood. In recent years there have been a number of attempts to correlate specific gene families or evolutionary processes with increased brain complexity in the vertebrate lineage. Candidates for evocation of complexity include genes involved in regulating brain size, such as neurotrophic factors or microcephaly-related genes; or wider evolutionary processes, such as accelerated evolution of brain-expressed genes or enhanced RNA splicing or editing events in primates. An inherent weakness of these studies is that they are correlative by nature, and almost exclusively focused on the mammalian and specifically the primate lineage. Another problem with genomic analyses is that it is difficult to identify functionally similar yet non-homologous molecules such as different families of cysteine-rich neurotrophic factors in different phyla. As long as comprehensive experimental studies of these questions are not feasible, additional perspectives for evolutionary and genomic studies will be very helpful. Cephalopod mollusks represent the most complex nervous systems outside the vertebrate lineage, thus we suggest that genome sequencing of different mollusk models will provide useful insights into the evolution of complex brains.

Regulatory roles of bone morphogenetic proteins and glucocorticoids in catecholamine production by rat pheochromocytoma cells

We here report a new physiological system that governs catecholamine synthesis involving bone morphogenetic proteins (BMPs) and activin in the rat pheochromocytoma cell line, PC12. BMP type I receptors, including activin receptor-like kinase-2 (ALK-2) (also referred to as ActRIA) and ALK-3 (BMPRIA), both type II receptors, ActRII and BMPRII, as well as the ligands BMP-2, -4, and -7 and inhibin/activin subunits were expressed in PC12 cells. PC12 cells predominantly secrete dopamine, whereas noradrenaline and adrenaline production is negligible. BMP-2, -4, -6, and -7 and activin A each suppressed dopamine and cAMP synthesis in a dose-dependent fashion. The BMP ligands also decreased 3,4-dihydroxyphenylalanine decarboxylase mRNA expression, whereas activin suppressed tyrosine hydroxylase expression. BMPs induced both Smad1/5/8 phosphorylation and Tlx2-Luc activation, whereas activin stimulated 3TP-Luc activity and p38 MAPK phosphorylation. ERK signaling was not affected by BMPs or activin. Dexamethasone enhanced catecholamine synthesis, accompanying increases in tyrosine hydroxylase and 3,4-dihydroxyphenylalanine decarboxylase transcription without cAMP accumulation. In the presence of dexamethasone, BMPs and activin failed to reduce dopamine as well as cAMP production. In addition, dexamethasone modulated mitotic suppression of PC12 induced by BMPs in a ligand-dependent manner. Furthermore, intracellular BMP signaling was markedly suppressed by dexamethasone treatment and the expression of ALK-2, ALK-3, and BMPRII was significantly inhibited by dexamethasone. Collectively, the endogenous BMP/activin system plays a key role in the regulation of catecholamine production. Controlling activity of the BMP system may be critical for glucocorticoid-induced catecholamine synthesis by adrenomedullar cells.