Evidence for a ligand-specific signaling through GFRalpha-1, but not GFRalpha-2, in the absence of Ret

A dual role for peripheral GDNF signaling in nociception and cardiovascular reflexes in the mouse

Group III/IV muscle afferents transduce nociceptive signals and modulate exercise pressor reflexes (EPRs). However, the mechanisms governing afferent responsiveness to dually modulate these processes are not well characterized. We and others have shown that ischemic injury can induce both nociception-related behaviors and exacerbated EPRs in the same mice. This correlated with primary muscle afferent sensitization and increased expression of glial cell line-derived neurotrophic factor (GDNF) in injured muscle and increased expression of GDNF family receptor α1 (GFRα1) in dorsal root ganglia (DRG). Here, we report that increased GDNF/GFRα1 signaling to sensory neurons from ischemia/reperfusion-affected muscle directly modulated nociceptive-like behaviors and increased exercise-mediated reflexes and group III/IV muscle afferent sensitization. This appeared to have taken effect through increased cyclic adenosine monophosphate (cAMP) response element binding (CREB)/CREB binding protein-mediated expression of the purinergic receptor P2X5 in the DRGs. Muscle GDNF signaling to neurons may, therefore, play an important dual role in nociception and sympathetic reflexes and could provide a therapeutic target for treating complications from ischemic injuries.

GFR-α1 Expression in Substantia Nigra Increases Bilaterally Following Unilateral Striatal GDNF in Aged Rats and Attenuates Nigral Tyrosine Hydroxylase Loss Following 6-OHDA Nigrostriatal Lesion

Glial cell line-derived neurotrophic factor (GDNF) improved motor function in Parkinson's disease (PD) patients in Phase I clinical trials, and these effects persisted months after GDNF discontinuation. Conversely, phase II clinical trials reported no significant effects on motor improvement vs placebo. The disease duration and the quantity, infusion approach, and duration of GDNF delivery may affect GDNF efficacy in PD treatment. However, identifying mechanisms activated by GDNF that affect nigrostriatal function may reveal additional avenues to partially restore nigrostriatal function. In PD and aging models, GDNF affects tyrosine hydroxylase (TH) expression or phosphorylation in substantia nigra (SN), long after a single GDNF injection in striatum. In aged rats, the GDNF family receptor, GFR-α1, increases TH expression and phosphorylation in SN. To determine if GFR-α1 could be a mechanistic link in long-term GDNF impact, we conducted two studies; first to determine if a single unilateral striatal delivery of GDNF affected GFR-α1 and TH over time (1 day, 1 week, and 4 weeks) in the striatum or SN in aged rats, and second, to determine if soluble GFR-α1 could mitigate TH loss following 6-hydroxydopamine (6-OHDA) lesion. In aged rats, GDNF bilaterally increased ser31 TH phosphorylation and GFR-α1 expression in SN at 1 day and 4 weeks after GDNF, respectively. In striatum, GFR-α1 expression decreased 1 week after GDNF, only on the GDNF-injected side. In 6-OHDA-lesioned rats, recombinant soluble GFR-α1 mitigated nigral, but not striatal, TH protein loss following 6-OHDA. Together, these results show GDNF has immediate and long-term impact on dopamine regulation in the SN, which includes a gradual increase in GFR-α1 expression that may sustain TH expression and dopamine function therein.

Ret-dependent and Ret-independent mechanisms of Gfl-induced sensitization

BACKGROUND

The GDNF family ligands (GFLs) are regulators of neurogenic inflammation and pain. We have previously shown that GFLs increase the release of the sensory neuron neuropeptide, calcitonin gene-related peptide (CGRP) from isolated mouse DRG.

RESULTS

Inhibitors of the mitogen-activated protein kinase (MAPK) pathway abolished the enhancement of CGRP release by GDNF. Neurturin-induced enhancement in the stimulated release of CGRP, used as an indication of sensory neuronal sensitization, was abolished by inhibition of the phosphatidylinositol-3 kinase (PI-3K) pathway. Reduction in Ret expression abolished the GDNF-induced sensitization, but did not fully inhibit the increase in stimulus-evoked release of CGRP caused by neurturin or artemin, indicating the presence of Ret-independent GFL-induced signaling in sensory neurons. Integrin β-1 and NCAM are involved in a component of Ret-independent GFL signaling in sensory neurons.

CONCLUSIONS

These data demonstrate the distinct and variable Ret-dependent and Ret-independent signaling mechanisms by which GFLs induce sensitization of sensory neurons. Additionally, there is a clear disconnect between intracellular signaling pathway activation and changes in sensory neuronal function.

Role of glial cell line-derived neurotrophic factor (GDNF)-neural cell adhesion molecule (NCAM) interactions in induction of neurite outgrowth and identification of a binding site for NCAM in the heel region of GDNF

The formation of appropriate neuronal circuits is an essential part of nervous system development and relies heavily on the outgrowth of axons and dendrites and their guidance to their respective targets. This process is governed by a large array of molecules, including glial cell line-derived neurotrophic factor (GDNF) and the neural cell adhesion molecule (NCAM), the interaction of which induce neurite outgrowth. In the present study the requirements for NCAM-mediated GDNF-induced neurite outgrowth were investigated in cultures of hippocampal neurons, which do not express Ret. We demonstrate that NCAM-mediated GDNF-induced signaling leading to neurite outgrowth is more complex than previously reported. It not only involves NCAM-140 and the Src family kinase Fyn but also uses NCAM-180 and the fibroblast growth factor receptor. We find that induction of neurite outgrowth by GDNF via NCAM or by trans-homophilic NCAM interactions are not mutually exclusive. However, whereas NCAM-induced neurite outgrowth primarily is mediated by NCAM-180, we demonstrate that GDNF-induced neurite outgrowth involves both NCAM-140 and NCAM-180. We also find that GDNF-induced neurite outgrowth via NCAM differs from NCAM-induced neurite outgrowth by being independent of NCAM polysialylation. Additionally, we investigated the structural basis for GDNF-NCAM interactions and find that NCAM Ig3 is necessary for GDNF binding. Furthermore, we identify within the heel region of GDNF a binding site for NCAM and demonstrate that a peptide encompassing this sequence mimics the effects of GDNF with regard to NCAM binding, activation of intracellular signaling, and induction of neurite outgrowth.

Driving GDNF expression: the green and the red traffic lights

Glial cell line-derived neurotrophic factor (GDNF) is widely recognized as a potent survival factor for dopaminergic neurons of the nigrostriatal pathway that degenerate in Parkinson's disease (PD). In animal models of PD, GDNF delivery to the striatum or the substantia nigra protects dopaminergic neurons against subsequent toxin-induced injury and rescues previously damaged neurons, promoting recovery of the motor function. Thus, GDNF was proposed as a potential therapy to PD aimed at slowing down, halting or reversing neurodegeneration, an issue addressed in previous reviews. However, the use of GDNF as a therapeutic agent for PD is hampered by the difficulty in delivering it to the brain. Another potential strategy is to stimulate the endogenous expression of GDNF, but in order to do that we need to understand how GDNF expression is regulated. The aim of this review is to do a comprehensive analysis of the state of the art on the control of endogenous GDNF expression in the nervous system, focusing mainly on the nigrostriatal pathway. We address the control of GDNF expression during development, in the adult brain and after injury, and how damaged neurons signal glial cells to up-regulate GDNF. Pharmacological agents or natural molecules that increase GDNF expression and show neuroprotective activity in animal models of PD are reviewed. We also provide an integrated overview of the signalling pathways linking receptors for these molecules to the induction of GDNF gene, which might also become targets for neuroprotective therapies in PD.

Chronic nerve compression injury induces a phenotypic switch of neurons within the dorsal root ganglia

Chronic nerve compression (CNC) injury initiates a series of pathological changes within the peripheral nerve at the site of injury. However, to date, little work has been performed to explore neuronal cell body responses to CNC injury. Here we show a preferential upregulation of growth-associated protein-43 (GAP-43) and enhanced Fluoro Ruby uptake by the small-diameter calcitonin gene-related protein (CGRP) and isolectin B4 (IB4)-positive neurons in the L4 and L5 ipsilateral dorsal root ganglion (DRG) 2 weeks and 1 month post injury. Furthermore, L4 and L5 DRGs ipsilateral to CNC injury also demonstrated a marked reduction in neurofilament 200 (NF-200) neurons and an increase in CGRP and IB4 neurons at early time points. All numbers normalized to values comparable to those of control when the DRG was evaluated 6 months post injury. Quantification of glial-derived neurotrophic factor (GDNF) protein revealed an upregulation in L4 and L5 DRG followed by a return to baseline values at later stages following injury. Upregulation of GDNF expression by Schwann cells was also readily apparent with both immunohistochemistry and Western blot analysis of 1 month compressed sciatic nerve specimens. Thus, CNC induces a phenotypic change in the DRG that appears to be temporally associated with increases in GDNF protein expression at and near the site of the compression injury in the nerve.

Survival, synaptogenesis, and regeneration of adult mouse spiral ganglion neurons in vitro

The inner ear spiral ganglion is populated by bipolar neurons connecting the peripheral sensory receptors, the hair cells, with central neurons in auditory brain stem nuclei. Hearing impairment is often a consequence of hair cell death, e.g., from acoustic trauma. When deprived of their peripheral targets, the spiral ganglion neurons (SGNs) progressively degenerate. For effective clinical treatment using cochlear prostheses, it is essential to maintain the SGN population. To investigate their survival dependence, synaptogenesis, and regenerative capacity, adult mouse SGNs were separated from hair cells and studied in vitro in the presence of various neurotrophins and growth factors. Coadministration of fibroblast growth factor 2 (FGF-2) and glial cell line-derived neurotrophic factor (GDNF) provided support for long-term survival, while FGF-2 alone could strongly promote neurite regeneration. Fibroblast growth factor receptor FGFR-3-IIIc was found to upregulate and translocate to the nucleus in surviving SGNs. Surviving SGNs formed contacts with other SGNs after they were deprived of the signals from the hair cells. In coculture experiments, neurites extending from SGNs projected toward hair cells. Interestingly, adult mouse spiral ganglion cells could carry out both symmetric and asymmetric cell division and give rise to new neurons. The authors propose that a combination of FGF-2 and GDNF could be an efficient route for clinical intervention of secondary degeneration of SGNs. The authors also demonstrate that the adult mammalian inner ear retains progenitor cells, which could commit neurogenesis.

The dependence receptor Ret induces apoptosis in somatotrophs through a Pit-1/p53 pathway, preventing tumor growth

Somatotrophs are the only pituitary cells that express Ret, GFRalpha1 and GDNF. This study investigated the effects of Ret in a somatotroph cell line, in primary pituitary cultures and in Ret KO mice. Ret regulates somatotroph numbers by inducing Pit-1 overexpression, leading to increased p53 expression and apoptosis, both of which can be prevented with Ret or Pit-1 siRNA. The Pit-1 overexpression is mediated by sustained activation of PKCdelta, JNK, c/EBPalpha and CREB induced by a complex of Ret, caspase 3 and PKCdelta. In the presence of GDNF, Akt is activated, and the Pit-1 overexpression and resulting apoptosis are blocked. The adenopituitary of Ret KO mice is larger than normal, showing Pit-1 and somatotroph hyperplasia. In normal animals, activation of the Ret/Pit-1/p53 pathway by retroviral introduction of Ret blocked tumor growth in vivo. Thus, somatotrophs have an intrinsic mechanism for controlling Pit-1/GH production through an apoptotic/survival pathway. Ret might be of value for treatment of pituitary adenomas.

Vascular-derived artemin: a determinant of vascular sympathetic innervation?

Vascular sympathetic innervation is an important determinant of blood pressure and blood flow. The mechanisms that determine vascular sympathetic innervation are not well understood. The present study tests the hypothesis that vascular-derived artemin promotes the development of sympathetic innervation to blood vessels by promoting sympathetic axon growth. RT-PCR and Western analyses indicate that artemin is expressed by cultured vascular smooth muscle and arteries, and artemin coreceptors, glial cell-derived neurotrophic factor family receptor alpha3 and ret, are expressed by postganglionic sympathetic neurons. The effects of artemin on axon growth were assessed on explants of neonatal rat sympathetic ganglia. In the presence, but not in the absence, of nerve growth factor, exogenous artemin stimulated neurite growth. Femoral arteries (FA) from adult rats contain artemin, and these arteries stimulated sympathetic neurite growth. Growth in the presence of FA was 92.2 +/- 11.9 mm, and that in the absence of FA was 26.3 +/- 5.4 mm (P < 0.05). FA stimulation of axon growth was reduced by an antibody that neutralized the activity of artemin (P < 0.05). These data indicate that artemin is expressed in arteries, and its receptors are expressed and functional in the postganglionic sympathetic neurons that innervate them. This suggests that artemin may be a determinant of vascular sympathetic innervation.

Intracerebral infusion of glial cell line-derived neurotrophic factor promotes striatal neurogenesis after stroke in adult rats

BACKGROUND AND PURPOSE

Stroke triggers increased progenitor proliferation in the subventricular zone (SVZ) and the generation of medium spiny neurons in the damaged striatum of rodents. We explored whether intrastriatal infusion of glial cell line-derived neurotrophic factor (GDNF) promotes neurogenesis after stroke.

METHODS

Adult rats were subjected to 2-hour middle cerebral artery occlusion (MCAO). GDNF was infused into the ischemic striatum either during the first week after MCAO, with the animals being killed directly thereafter, or during the third and fourth weeks, with the rats being killed 1 week later. New cells were labeled with 5'-bromo-2'deoxyuridine (BrdU) on day 7 or during the second week, respectively. Neurogenesis was assessed immunocytochemically with antibodies against BrdU and neuronal, glial, or progenitor markers. GDNF receptor expression was analyzed in SVZ tissue and neurospheres by reverse transcription-polymerase chain reaction and immunocytochemistry.

RESULTS

GDNF infusion increased cell proliferation in the ipsilateral SVZ and the recruitment of new neuroblasts into the striatum after MCAO and improved survival of new mature neurons. The GDNF receptor GFRalpha1 was upregulated in the SVZ 1 week after MCAO and was coexpressed with markers of dividing progenitor cells.

CONCLUSIONS

Intrastriatal infusion of GDNF in the postischemic period promotes several steps of striatal neurogenesis after stroke, partly through direct action on SVZ progenitors. Because delivery of GDNF has biological effects in the human brain, our data suggest that administration of this factor may promote neuroregenerative responses in stroke patients.

GDNF family ligand receptor components Ret and GFRalpha-1 in the human trigeminal ganglion and sensory nuclei

The occurrence of Ret and GFRalpha-1 receptors is shown by immunohistochemistry in the human trigeminal sensory system at pre-, postnatal and adult age. Receptor-labeled neurons occur in both trigeminal ganglion and mesencephalic nucleus. In adult trigeminal ganglion, about 75% of Ret- and 65% of GFRalpha-1-labeled neurons are small- and medium-sized. The proportion of Ret+ and GFRalpha-1+ trigeminal ganglion neurons in the adult is about 25 and 60%, respectively. The majority of Ret+ are double labeled for GFRalpha-1 and glial cell line-derived neurotrophic factor (GDNF). Most of the GFRalpha-1+ cells contain GDNF and about 50% of them contain Ret. Triple labeling shows many Ret+/GDNF+/GFRalpha-1+ neurons, but also a number of Ret-/GDNF+/GFRalpha-1+ and Ret+/GDNF-/GFRalpha-1+ cells. Both Ret+ and GFRalpha-1+ neuronal subpopulations overlap with that containing calcitonin gene-related peptide. Ret+ pericellular basket-like nerve fibers occur in the adult trigeminal ganglion. Centrally, immunoreactivity is restricted to the spinal nucleus pars caudalis and pars interpolaris and to the mesencephalic nucleus. In adult specimens, Ret+ nerve fibers and puncta gather in the inner substantia gelatinosa. Ret+ neurons occur in the spinal nucleus and are more frequent in newborn than in adult subjects. Central GFRalpha-1+-labeled neurons and punctate elements are sparse. These findings support the involvement of GDNF and possibly other cognate ligands in the trophism of human trigeminal primary sensory neurons from prenatal life to adulthood, indicating a selective commitment to cells devoted to protopathic and proprioceptive sensory transmission. They also support the possibility that receptor molecules other than Ret could be active in transducing the ligand signal.

Tissue expression of alternatively spliced GFRα1, NCAM and RET isoforms and the distinct functional consequence of ligand-induced activation of GFRα1 isoforms

Glial-cell-line-derived neurotrophic factor (GDNF) exerts its effect through a multi-component receptor system consisting of GFRalpha1, RET and NCAM. Two highly homologous alternatively spliced GFRalpha1 isoforms (GFRalpha1a and GFRalpha1b) have previously been identified. In this study, isoform specific real-time PCR assays were used to quantify the expression levels of GFRalpha1, RET and NCAM isoforms in murine embryonic and adult tissues. The expression levels of GFRalpha1b were found to be comparable to that of GFRalpha1a in peripheral tissues. However, GFRalpha1a was the predominant isoform expressed in the whole brain. The co-expressions of GFRalpha1 and the co-receptors were developmentally regulated and differentially expressed in some tissues. Microarray analyses of GFRalpha1 isoforms transfected cells stimulated with NTN showed distinct and non-overlapping gene profiles. These observations are consistent with the emerging view that the combinatorial interactions of the spliced isoforms of GFRalpha, RET and NCAM may contribute to the pleiotropic biological responses.

Involvement of glial cell line-derived neurotrophic factor in activation processes of rodent macrophages

The physiological roles of glial cell line-derived neurotrophic factor (GDNF) expressed in the microglia/macrophages of the injured spinal cord have not yet been clarified. mRNA expression of chemokines, including monocyte chemoattractant protein (MCP)-1, was evoked within 1 hr after transection of the spinal cord, and GDNF mRNA expression was similarly up-regulated. Immunohistochemical analysis showed that GDNF was coexpressed with MCP-1 in the CD11b-positive cells. Therefore, we examined further the effects of GDNF on cultured rat peritoneal macrophages. GDNF enhanced the phagocytic activity of the macrophages via GFRalpha-1, glycosylphosphatidylinositol-anchored specific binding site of GDNF, in a c-Ret-independent manner. The influence of autocrine and/or paracrine GDNF synthesis was evaluated by performing activation experiments using macrophages cultured from heterozygous (+/-) GDNF gene-deficient mice or wild-type (+/+) mice. There were no morphological differences dependent on genetic types or stimulators. However, the GDNF mRNA level, but not the MCP-1 or GFRalpha-1 mRNA level, was substantially lower in the mutant macrophages than in the +/+ cells irrespective of stimulation with MCP-1 or lipopolysaccharide (LPS). The phagocytic activity enhanced by MCP-1 or LPS was significantly lower in the mutant cells (+/-) than in the +/+ ones, demonstrating the involvement of endogenous GDNF in the activation processes of macrophages in vitro and suggesting that not only neuroprotective function but also activation of macrophages is effected by the GDNF produced after a spinal cord injury.

What turns CREB on?

The transactivation domain of the cAMP response element-binding protein (CREB) consists of two major domains. The glutamine-rich Q2 domain, which interacts with the general transcription factor TAFII130/135, is sufficient for the recruitment of a functional RNA polymerase II complex and allows basal transcriptional activity. The kinase-inducible domain, however, mediates signal-induced activation of CREB-mediated transcription. It is generally believed that recruitment of the coactivators CREB-binding protein (CBP) and p300 after signal-induced phosphorylation of this domain at serine-133 strongly enhances CREB-dependent transcription. Transcriptional activity of CREB can also be potentiated by phosphoserine-133-independent mechanisms, and not all stimuli that provoke phosphorylation of serine-133 stimulate CREB-dependent transcription. This review presents an overview of the diversity of stimuli that induce CREB phosphorylation at Ser-133, focuses on phosphoserine-133-dependent and -independent mechanisms that affect CREB-mediated transcription, and discusses different models that may explain the discrepancy between CREB Ser-133 phosphorylation and activation of CREB-mediated transcription.

GDNF elicits distinct immediate-early gene responses in cultured cortical and mesencephalic neurons

Glial cell line-derived neurotrophic factor (GDNF) has been recognized as a survival-promoting molecule for several neuronal populations in the central nervous system (CNS), including midbrain dopaminergic neurons and cortical neurons. Whereas it is well established that GDNF affects dopaminergic cell survival through a receptor complex composed of the tyrosine kinase, Ret, and the glycosylphosphatidylinositol (GPI)-anchored protein, GFRalpha-1, c-Ret is basically undetectable in cortical neurons. In the present study, we have compared GDNF signaling in cortical and mesencephalic neurons by using GDNF-induced expression of the immediate-early genes, c-fos and mgif, as a readout. We found that stimulation of embryonic day (E)17 cortical cultures for 3 hr with GDNF at concentrations ranging from 10 to 80 ng/ml did not result in detectable c-fos expression. In contrast, c-fos expression occurred in E14 mesencephalic cultures exposed to both low and high GDNF concentrations. Vice versa, cortical neurons responded to high GDNF concentrations (80 ng/ml) with an increase in mRNA encoding mGIF, while a similar mGIF response was absent in mesencephalic cultures. Cleavage of GFRalpha receptor subunits from their GPI anchors by phosphatidylinositol-specific phospholipase C (PIPLC) abolished GDNF-induced c-fos expression in mesencephalic cultures, but did not interfere with the effects of GDNF on cortical mgif expression. Together, these findings point to distinct differences in the GDNF recognition and/or signal transduction machinery of cortical and mesencephalic neurons.

Internalization of glial cell-derived neurotrophic factor receptor GFR alpha 1 in the absence of the ret tyrosine kinase coreceptor

1. Glial cell-derived neurothrophic factor (GDNF) interacts with a cell surface receptor, GFRalpha1, that is linked via a glycosyl-phosphatidylinositol (GPI) lipid to the cell membrane. The neurotrophic activities of GDNF are mediated by binding to GFRalpha1 and further interaction of the GDNF-GFRalpha1 complex with a coreceptor tyrosine kinase encoded by the c-Ret protooncogene. There is also evidence for the existence of cell signaling by GDNF and GFRalpha1 in the absence of Ret. 2. To further delineate the Ret-dependent and -independent functions of GDNF, cellular internalization of GDNF and GFRalpha1 was examined in cells lines and primary neurons. 3. Relative to other GPI-anchored receptors, efficient endocytosis (approximately 30-40% of total surface-bound ligand internalized after 2 min) of GNDF and GFRalpha1 was observed in neuroblastoma and transfected-fibroblast cell lines that lack Ret. Primary hippocampal neurons from transgenic mice that express a wild-type GFRalpha1 together with a mutant, tyrosine kinase-inactive Ret also internalized GDNF efficiently (approximately 20% of total surface-bound ligand internalized after 2 min). We also observed a ligand dependence for GFRalpha1 internalization in the cell lines that lack Ret. Furthermore, a comparison in the presence and absence of Ret indicates that this coreceptor tyrosine kinase slows internalization at early time points. 4. The data suggest different mechanisms of internalization for GDNF-GFRalpha1 in the absence and presence of the Ret coreceptor.

Glial cell line-derived neurotrophic factor alters axon schwann cell units and promotes myelination in unmyelinated nerve fibers

Glial cell line-derived neurotrophic factor (GDNF) plays an important role in the development and maintenance of a subset of dorsal root ganglion sensory neurons. We administered high-dose exogenous recombinant human GDNF (rhGDNF) daily to adult rats to examine its effect on unmyelinated axon-Schwann cell units in intact peripheral nerves. In rhGDNF-treated animals, there was a dramatic proliferation in the Schwann cells of unmyelinated fibers, which resulted in the segregation of many unmyelinated axons into a 1:1 relationship with Schwann cells and myelination of normally unmyelinated small axons. This study demonstrates that the administration of high doses of a growth factor to adult rats can change the phenotype of nerve fibers from unmyelinated to myelinated.

The GDNF family: signalling, biological functions and therapeutic value

Members of the nerve growth factor (NGF) and glial cell line-derived neurotrophic factor (GDNF) families comprising neurotrophins and GDNF-family ligands (GFLs), respectively are crucial for the development and maintenance of distinct sets of central and peripheral neurons. Knockout studies in the mouse have revealed that members of these two families might collaborate or act sequentially in a given neuron. Although neurotrophins and GFLs activate common intracellular signalling pathways through their receptor tyrosine kinases, several clear differences exist between these families of trophic factors.