Enhancement of Tyrosine Hydroxylase Phosphorylation and Activity by Glial Cell Line-derived Neurotrophic Factor

Tyrosine hydroxylase inhibits HCC progression by downregulating TGFβ/Smad signaling

The alteration of metabolic processes has been found to have significant impacts on the development of hepatocellular carcinoma (HCC). Nevertheless, the effects of dysfunction of tyrosine metabolism on the development of HCC remains to be discovered. This research demonstrated that tyrosine hydroxylase (TH), which responsible for the initial and limiting step in the bio-generation of the neuro-transmitters dopamine and adrenaline, et al. was shown to be reduced in HCC. Increased expression of TH was found facilitates the survival of HCC patients. In addition, decreased TH indicated larger tumor size, much more numbers of tumor, higher level of AFP, and the presence of cirrhosis. TH effectively impairs the growth and metastasis of HCC cells, a process dependent on the phosphorylation of serine residues (S19/S40). TH directly binds to Smad2 and hinders the cascade activation of TGFβ/Smad signaling with the treatment of TGFβ1. In summary, our study uncovered the non-metabolic functions of TH in the development of HCC and proposes that TH might be a promising biomarker for diagnosis as well as an innovative target for metastatic HCC.

Physical Exercise Promotes Beneficial Changes on Neurotrophic Factors in Mesolimbic Brain Areas After AMPH Relapse: Involvement of the Endogenous Opioid System

Addiction is a serious public health problem, and the current pharmacotherapy is unable to prevent drug use reinstatement. Studies have focused on physical exercise as a promising coadjuvant treatment. Our research group recently showed beneficial neuroadaptations in the dopaminergic system related to amphetamine-relapse prevention involving physical exercise-induced endogenous opioid system activation (EXE-OS activation). In this context, additional mechanisms were explored to understand the exercise benefits on drug addiction. Male rats previously exposed to amphetamine (AMPH, 4.0 mg/kg) for 8 days were submitted to physical exercise for 5 weeks. EXE-OS activation was blocked by naloxone administration (0.3 mg/kg) 5 min before each physical exercise session. After the exercise protocol, the rats were re-exposed to AMPH for 3 days, and in sequence, euthanasia was performed and the VTA and NAc were dissected. In the VTA, our findings showed increased immunocontent of proBDNF, BDNF, and GDNF and decreased levels of AMPH-induced TrkB; therefore, EXE-OS activation increased all these markers and naloxone administration prevented this exercise-induced effect. In the NAc, the same molecular markers were also increased by AMPH and decreased by EXE-OS activation. In this study, we propose a close relation between EXE-OS activation beneficial influence and a consequent neuroadaptation on neurotrophins and dopaminergic system levels in the mesolimbic brain area, preventing the observed AMPH-relapse behavior. Our outcomes bring additional knowledge concerning addiction neurobiology understanding and show that EXE-OS activation may be a potential adjuvant tool in drug addiction therapy.

Tyrosine hydroxylase phosphorylation is under the control of serine 40

Tyrosine hydroxylase catalyzes the initial and rate-limiting step in the biosynthesis of the neurotransmitter dopamine. The phosphorylation state of Ser40 and Ser31 is believed to exert a direct effect on the enzymatic activity of tyrosine hydroxylase. Interestingly, some studies report that Ser31 phosphorylation affects Ser40 phosphorylation, while Ser40 phosphorylation has no effect on Ser31 phosphorylation, a process named hierarchical phosphorylation. Here, we provide a detailed investigation into the signal transduction mechanisms regulating Ser40 and Ser31 phosphorylation in dopaminergic mouse MN9D and Neuro2A cells. We find that cyclic nucleotide signaling drives Ser40 phosphorylation, and that Ser31 phosphorylation is strongly regulated by ERK signaling. Inhibition of ERK1/2 with UO126 or PD98059 reduced Ser31 phosphorylation, but surprisingly had no effect on Ser40 phosphorylation, contradicting a role for Ser31 in the regulation of Ser40. Moreover, to elucidate a possible hierarchical mechanism controlling tyrosine hydroxylase phosphorylation, we introduced tyrosine hydroxylase variants in Neuro2A mouse neuroblastoma cells that mimic either phosphorylated or unphosphorylated serine residues. When we introduced a Ser40Ala tyrosine hydroxylase variant, Ser31 phosphorylation was completely absent. Additionally, neither the tyrosine hydroxylase variant Ser31Asp, nor the variant Ser31Ala had any significant effect on basal Ser40 phosphorylation levels. These results suggest that tyrosine hydroxylase is not controlled by hierarchical phosphorylation in the sense that first Ser31 has to be phosphorylated and subsequently Ser40, but, conversely, that Ser40 phosphorylation is essential for Ser31 phosphorylation. Overall our study suggests that Ser40 is the crucial residue to target so as to modulate tyrosine hydroxylase activity.

Finding an Optimal Level of GDNF Overexpression: Insights from Dopamine Cycling

The application of glial cell line-derive neurotrophic factor (GDNF) to cell cultures and animal models has demonstrated positive effects upon dopaminergic neuronal survival and development, function, restoration, and protection. On this basis, recombinant GDNF protein has been trialled in the treatment of late-stage human Parkinson's disease patients with only limited success that is likely due to a lack of viable receptor targets in an advanced state of neurodegeneration. The latest research points to more refined approaches of modulating GDNF signalling and an optimal quantity and spatial regulation of GDNF can be extrapolated using regulation of dopamine as a proxy measure. The basic research literature on dopaminergic effects of GDNF in animal models is reviewed, concluding that a twofold increase in natively expressing cells increases dopamine turnover and maximises neuroprotective and beneficial motor effects whilst minimising hyperdopaminergia and other side-effects. Methodological considerations for measurement of dopamine levels and neuroanatomical distinctions are made between populations of dopamine neurons and their respective effects upon movement and behaviour that will inform future research into this still-relevant growth factor.

Mesencephalic Astrocyte-Derived Neurotrophic Factor (MANF) Elevates Stimulus-Evoked Release of Dopamine in Freely-Moving Rats

Neurotrophic factors (NTFs) hold potential as disease-modifying therapies for neurodegenerative disorders like Parkinson's disease. Glial cell line-derived neurotrophic factor (GDNF), cerebral dopamine neurotrophic factor (CDNF), and mesencephalic astrocyte-derived neurotrophic factor (MANF) have shown neuroprotective and restorative effects on nigral dopaminergic neurons in various animal models of Parkinson's disease. To date, however, their effects on brain neurochemistry have not been compared using in vivo microdialysis. We measured extracellular concentration of dopamine and activity of dopamine neurochemistry-regulating enzymes in the nigrostriatal system of rat brain. NTFs were unilaterally injected into the striatum of intact Wistar rats. Brain microdialysis experiments were performed 1 and 3 weeks later in freely-moving animals. One week after the treatment, we observed enhanced stimulus-evoked release of dopamine in the striatum of MANF-treated rats, but not in rats treated with GDNF or CDNF. MANF also increased dopamine turnover. Although GDNF did not affect the extracellular level of dopamine, we found significantly elevated tyrosine hydroxylase (TH) and catechol-O-methyltransferase (COMT) activity and decreased monoamine oxidase A (MAO-A) activity in striatal tissue samples 1 week after GDNF injection. The results show that GDNF, CDNF, and MANF have divergent effects on dopaminergic neurotransmission, as well as on dopamine synthetizing and metabolizing enzymes. Although the cellular mechanisms remain to be clarified, knowing the biological effects of exogenously administrated NTFs in intact brain is an important step towards developing novel neurotrophic treatments for degenerative brain diseases.

Applications of digital holographic microscopy in therapeutic evaluation of Chinese herbal medicines

Therapeutic use of Chinese herbal medicines (CHMs) is a new approach to treat neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease. The detection of soma volume and neurite outgrowth of living neurons is a highly relevant biomarker related to various application fields, including therapy efficacy and drug safety evaluation. Through the use of digital holographic microscopy (DHM), we may evaluate the therapeutic effect of CHMs in curing neurodegeneration. Panax ginseng has been used in traditional Chinese herbal medicine for centuries. In this study, DHM is applied to monitor the three-dimensional morphology change of retinoic acid-induced human neuroblastoma SH-SY5Y cells during Panax ginseng treatment. We demonstrate the capability of DHM to detect noninvasively SH-SY5Y cell apoptosis and rescue through the measurement of neuronal volume and neurite outgrowth regulation without any labeling reagent. Through DHM, we observed the phase images of the rapidly shrinking cells with decreasing soma volume and shortening neurite outgrowth during glutamate treatments. Then shrinkage in glutamate-induced cells is significantly alleviated during Panax ginseng treatment. The results through DHM are consistent with the result from MTT assay for assessing cell viability during Panax ginseng treatment. Thus, we suggest that application of DHM for measuring soma volume and neurite outgrowth of living neurons may be one appropriate therapeutic evaluation for CHMs.

Retinoic acid protects against proteasome inhibition associated cell death in SH-SY5Y cells via the AKT pathway

Inhibition of proteasome activity and the resulting protein accumulation are now known to be important events in the development of many neurological disorders, including Alzheimer's and Parkinson's diseases. Abnormal or over expressed proteins cause endoplasmic reticulum and oxidative stress leading to cell death, thus, normal proteasome function is critical for their removal. We have shown previously, with cultured SH-SY5Y neuroblastoma cells, that proteasome inhibition by the drug epoxomicin results in accumulation of ubiquitinated proteins. This causes obligatory loading of the mitochondria with calcium (Ca(2+)), resulting in mitochondrial damage and cytochrome c release, followed by programmed cell death (PCD). In the present study, we demonstrate that all-trans-retinoic acid (RA) pretreatment of SH-SY5Y cells protects them from PCD death after subsequent epoxomicin treatment which causes proteasome inhibition. Even though ubiquitinated protein aggregates are present, there is no evidence to suggest that autophagy is involved. We conclude that protection by RA is likely by mechanisms that interfere with cell stress-PCD pathway that otherwise would result from protein accumulation after proteasome inhibition. In addition, although RA activates both the AKT and ERK phosphorylation signaling pathways, only pretreatment with LY294002, an inhibitor of PI3-kinase in the AKT pathway, removed the protective effect of RA from the cells. This finding implies that RA activation of the AKT signaling cascade takes precedence over its activation of ERK1/2 phosphorylation, and that this selective effect of RA is key to its protection of epoxomicin-treated cells. Taken together, these findings suggest that RA treatment of cultured neuroblastoma cells sets up conditions under which proteasome inhibition, and the resultant accumulation of ubiquitinated proteins, loses its ability to kill the cells and may likely play a therapeutic role in neurodegenerative diseases.

Chronic ketamine administration modulates midbrain dopamine system in mice

Ketamine is an anesthetic and a popular abusive drug. As an anesthetic, effects of ketamine on glutamate and GABA transmission have been well documented but little is known about its long-term effects on the dopamine system. In the present study, the effects of ketamine on dopamine were studied in vitro and in vivo. In pheochromocytoma (PC 12) cells and NGF differentiated-PC 12 cells, ketamine decreased the cell viability while increasing dopamine (DA) concentrations in a dose-related manner. However, ketamine did not affect the expression of genes involved in DA synthesis. In the long-term (3 months) ketamine treated mice, significant increases of DA contents were found in the midbrain. Increased DA concentrations were further supported by up-regulation of tyrosine hydroxylase (TH), the rate limiting enzyme in catecholamine synthesis. Activation of midbrain dopaminergic neurons could be related to ketamine modulated cortical-subcortical glutamate connections. Using western blotting, significant increases in BDNF protein levels were found in the midbrain, suggesting that perhaps BDNF pathways in the cortical-subcortical connections might contribute to the long-term ketamine induced TH upregulation. These data suggest that long-term ketamine abuse caused a delayed and persistent upregulation of subcortical DA systems, which may contribute to the altered mental status in ketamine abusers.

Effects of intrastriatal GDNF on the response of dopamine neurons to 6-hydroxydopamine: time course of protection and neurorestoration

Glial cell line-derived neurotrophic factor (GDNF) protects dopamine (DA) neurons from 6-hydroxydopamine (6-OHDA) toxicity. We have now explored this protection over 8 weeks following toxin administration. Infusion of Fluoro-Gold (FG) into the striatum was followed 1 week later by GDNF (9μg) or its vehicle. Six hours later, animals received 6-OHDA (4 μg) into the same site. 6-OHDA caused a loss of cells in the substantia nigra that expressed both FG and tyrosine hydroxylase (TH) and striatal terminals expressing TH, the high affinity dopamine transporter (DAT), and the vesicular monoamine transporter 2 (VMAT2) as assessed 2-8 weeks later. Loss of FG(+) cells, and striatal DA was completely blocked by GDNF by 2 weeks. In contrast, GDNF only slightly attenuated the loss of TH, DAT, or VMAT2 in the striatum at 2 weeks, but had restored these markers by 4-8 weeks. Thus, GDNF prevents DA cell death and loss of striatal DA content, but several weeks are required to fully restore the dopaminergic phenotype. These results provide insight into the mechanism of GDNF protection of DA neurons, and may help avoid incorrect interpretations of temporary phenotypic changes.

Nucleus accumbens-derived glial cell line-derived neurotrophic factor is a retrograde enhancer of dopaminergic tone in the mesocorticolimbic system

Spontaneous firing of ventral tegmental area (VTA) dopamine (DA) neurons provides ambient levels of DA in target areas such as the nucleus accumbens (NAc) and the prefrontal cortex (PFC). Here we report that the glial cell line-derived neurotrophic factor (GDNF), produced in one target region, the NAc, is retrogradely transported by DA neurons to the VTA where the growth factor positively regulates the spontaneous firing activity of both NAc- and PFC-projecting DA neurons in a mechanism that requires the activation of the mitogen-activated protein kinase (MAPK) pathway. We also show that the consequence of GDNF-mediated activation of the MAPK signaling cascade in the VTA is an increase in DA overflow in the NAc. Together, these results demonstrate that NAc-produced GDNF serves as a retrograde enhancer that upregulates the activity of the mesocorticolimbic DA system.

Altered adrenergic receptor signaling following traumatic brain injury contributes to working memory dysfunction

The prefrontal cortex is highly vulnerable to traumatic brain injury (TBI) and its structural and/or functional alterations as a result of TBI can give rise to persistent working memory (WM) dysfunction. Using a rodent model of TBI, we have described profound WM deficits following TBI that are associated with increases in prefrontal catecholamine (both dopamine and norepinephrine) content. In this study, we examined if enhanced norepinephrine signaling contributes to TBI-associated WM dysfunction. We demonstrate that administration of α1 adrenoceptor antagonists, but not α2A agonist, at 14 days post-injury significantly improved WM performance. mRNA analysis revealed increased levels of α1A, but not α1B or α1D, adrenoceptor in the medial prefrontal cortex (mPFC) of brain-injured rats. As α1A and 1B adrenoceptor promoters contain putative cAMP response element (CRE) sequences, we therefore examined if CRE-binding protein (CREB) actively engages these sequences in order to increase receptor gene transcription following TBI. Our results show that the phosphorylation of CREB is enhanced in the mPFC at time points during which increased α1A mRNA expression was observed. Chromatin immunoprecipitation (ChIP) assays using mPFC tissue from injured animals indicated increased phospho-CREB binding to the CRE sites of α1A, but not α1B, promoter compared to that observed in uninjured controls. To address the translatability of our findings, we tested the efficacy of the FDA-approved α1 antagonist Prazosin and observed that this drug improves WM in injured animals. Taken together, these studies suggest that enhanced CREB-mediated expression of α1 adrenoceptor contributes to TBI-associated WM dysfunction, and therapies aimed at reducing α1 signaling may be useful in the treatment of TBI-associated WM deficits in humans.

Initiation of dopaminergic differentiation of Nurr1(-) mesencephalic precursor cells depends on activation of multiple mitogen-activated protein kinase pathways

Interleukin-1 (IL-1) plays a pivotal role in terminal dopaminergic differentiation of midbrain-derived neural precursor cells already committed to the mesencephalic dopaminergic phenotype (named mdNPCs for mesencephalic dopaminergic neural precursor cells). Here we characterized the molecular events in long-term expanded rat nuclear receptor related-1(-) (Nurr1(-)) mdNPCs in response to IL-1beta during their terminal dopaminergic specification. We showed that IL-1beta induced a rapid induction of mRNA of dopaminergic key fate-determining transcription factors, such as Nurr1 and Pitx3, and a subsequent increase of tyrosine hydroxylase protein as an early marker for dopaminergic neurons in vitro. These effects of IL-1beta were specific for mdNPCs and were not observed in striatal neural precursor cells (NPCs). Surprisingly, IL-1beta did not activate the NF-kappaB pathway or the transcription factor activating protein 1 (AP-1), but inhibition of nuclear translocation of NF-kappaB by SN50 facilitated IL-1beta-induced Nurr1 expression and dopaminergic differentiation of mdNPCs. Incubation of mdNPCs with IL-1beta led to a rapid phosphorylation of ERK1/2 and p38 mitogen-activated protein (MAP) kinases within 1 to 3 hours, whereas Jun kinase was not phosphorylated in response to IL-1beta. Consistently, inhibition of the ERK1/2 pathway or p38 MAP kinase blocked Nurr1 upregulation and further dopaminergic specification of mdNPCs, but not differentiation into MAP2ab(+) neurons. IL-1 receptor antagonist did not block early dopaminergic differentiation events, suggesting that the effects of IL-1beta are not mediated through activation of IL-1 receptor type I. Our results indicate that induction of terminal dopaminergic specification of Nurr1(-) mdNPCs by IL-1beta depends on activation of the ERK1/2 and p38 MAP kinase pathway.

Differentiation of mouse Neuro 2A cells into dopamine neurons

Neuro 2A (N2a) is a mouse neural crest-derived cell line that has been extensively used to study neuronal differentiation, axonal growth and signaling pathways. A convenient characteristic of these cells is their ability to differentiate into neurons within a few days. However, most differentiation methods reported for N2a cells do not provide information about the neuronal types obtained after each treatment. In this study, we evaluated the generation of N2a dopamine neurons following treatment with a number of factors known to induce neuronal differentiation. Our results showed that N2a cells express Nurr-related factor 1 (Nurr1) and produce low levels of tyrosine hydroxylase (TH) and dopamine. Both TH and dopamine levels were significantly enhanced in the presence of dibutyryl cyclic adenosine monophosphate (dbcAMP), as evidenced by Western blot, immunocytochemistry and high performance liquid chromatography (HPLC). In contrast to dbcAMP, other factors such as transforming growth factor beta1 (TGF beta 1), bone morphogenetic protein 4 (BMP4), glial cell-derived neurotrophic factor (GDNF) and retinoic acid (RA) did not increase TH expression. Further investigation confirmed that the effect of dbcAMP on production of TH-positive neurons was mediated through cyclic AMP (cAMP) responsive element binding protein (CREB) and it was antagonized by RA. Thus, although various treatments can be used to generate N2a neurons, only dbcAMP significantly enhanced the formation of dopamine neurons. Taken together, this study provided a simple and reliable method to generate dopamine neurons for rapid and efficient physiological and pharmacological assays.

Excessive alcohol consumption is blocked by glial cell line-derived neurotrophic factor

We previously found that activation of the glial cell line-derived neurotrophic factor (GDNF) pathway in the ventral tegmental area (VTA) reduces moderate alcohol (ethanol) intake in a rat operant self-administration paradigm. Here, we set out to assess the effect of GDNF in the VTA on excessive voluntary consumption of ethanol. Long-Evans rats were trained to drink large quantities of a 20% ethanol solution in an intermittent-access two-bottle choice drinking paradigm. The rats were given three 24-h sessions per week, and GDNF's actions were measured when rats achieved a baseline of ethanol consumption of 5.5g/kg/24h. We found that microinjection of GDNF into the VTA 10min before the beginning of an ethanol-drinking session significantly reduced ethanol intake and preference, but did not affect total fluid intake. We further show that GDNF greatly decreased both the first bout of excessive ethanol intake at the beginning of the session, and the later consummatory activity occurring during the dark cycle. These data suggest that GDNF is a rapid and long-lasting inhibitor of "binge-like" ethanol consumption.

GDNF--a potential target to treat addiction

The glial cell line-derived neurotrophic factor (GDNF) is a secreted protein, best known for its role in the development of the central and peripheral nervous systems and the survival of adult dopaminergic neurons. More recently, accumulating evidence suggests that GDNF plays a unique role in negatively regulating the actions of drugs of abuse. In this article, we review these data and highlight the possibility that the GDNF pathway may be a promising target for the treatment of addiction.

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.

GDNF is a fast-acting potent inhibitor of alcohol consumption and relapse

Previously, we demonstrated that the action of the natural alkaloid, ibogaine, to reduce alcohol (ethanol) consumption is mediated by the glial cell line-derived neurotrophic factor (GDNF) in the ventral tegmental area (VTA). Here we set out to test the actions of GDNF in the VTA on ethanol-drinking behaviors. We found that GDNF infusion very rapidly and dose-dependently reduced rat ethanol, but not sucrose, operant self-administration. A GDNF-mediated decrease in ethanol consumption was also observed in rats with a history of high voluntary ethanol intake. We found that the action of GDNF on ethanol consumption was specific to the VTA as infusion of the growth factor into the neighboring substantia nigra did not affect operant responses for ethanol. We further show that intra-VTA GDNF administration rapidly activated the MAPK signaling pathway in the VTA and that inhibition of the MAPK pathway in the VTA blocked the reduction of ethanol self-administration by GDNF. Importantly, we demonstrate that GDNF infused into the VTA alters rats' responses in a model of relapse. Specifically, GDNF application blocked reacquisition of ethanol self-administration after extinction. Together, these results suggest that GDNF, via activation of the MAPK pathway, is a fast-acting selective agent to reduce the motivation to consume and seek alcohol.

Glial cell-derived neurotrophic factor protects against proteasome inhibition-induced dopamine neuron degeneration by suppression of endoplasmic reticulum stress and caspase-3 activation

Evidence has shown that ubiquitin proteasome system (UPS) impairment plays an important role in the dopamine (DA) neurodegeneration in Parkinson's disease (PD). It has been reported that application of proteasomal inhibitor lactacystin in ventral mesencephalon (VM) cultures can cause DA neurodegeneration, although the underlying mechanisms are not clear. Herein, we used the lactacystin-induced DA cell degeneration model to study the neuroprotection of glial cell-derived neurotrophic factor (GDNF) in VM cultures. We measured the expression of endoplasmic reticulum stress (ERS)-related genes, and determined the caspase-3 activation, apoptotic cell death, as well as alpha-synuclein-positive inclusions in DA neurons. We found that GDNF treatment significantly suppressed the expression of ERS-related genes and inhibited the activation of caspase-3 and apoptotic cell death without affecting alpha-synuclein-positive inclusions in DA neurons. Our study suggests that the protection of GDNF against DA neurodegeneration in the UPS impairment model is associated with ERS and caspase-3 suppression.

Glial cell line-derived neurotrophic factor and neurturin inhibit neurite outgrowth and activate RhoA through GFR alpha 2b, an alternatively spliced isoform of GFR alpha 2

The glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) belong to a structurally related family of neurotrophic factors. NTN exerts its effect through a multicomponent receptor system consisting of the GDNF family receptor alpha2 (GFR alpha2), RET, and/or NCAM (neural cell adhesion molecule). GFR alpha2 is alternatively spliced into at least three isoforms (GFR alpha2a, GFR alpha2b, and GFR alpha2c). It is currently unknown whether these isoforms share similar functional and biochemical properties. Using highly specific and sensitive quantitative real-time PCR, these isoforms were found to be expressed at comparable levels in various regions of the human brain. When stimulated with GDNF and NTN, both GFR alpha2a and GFR alpha2c, but not GFR alpha2b, promoted neurite outgrowth in transfected Neuro2A cells. These isoforms showed ligand selectivity in MAPK (mitogen-activated protein kinase) [ERK1/2 (extracellular signal-regulated kinase 1/2)] and Akt signaling. In addition, the GFR alpha2 isoforms regulated different early-response genes when stimulated with GDNF or NTN. In coexpression studies, GFR alpha2b was found to inhibit ligand-induced neurite outgrowth by GFR alpha2a and GFR alpha2c. Stimulation of GFR alpha2b also inhibited the neurite outgrowth induced by GFR alpha1a, another member of the GFR alpha. Furthermore, activation of GFR alpha2b inhibited neurite outgrowth induced by retinoic acid and activated RhoA. Together, these data suggest a novel paradigm for the regulation of growth factor signaling and neurite outgrowth via an inhibitory splice variant of the receptor. Thus, depending on the expressions of specific GFR alpha2 receptor spliced isoforms, GDNF and NTN may promote or inhibit neurite outgrowth through the multicomponent receptor complex.

Constitutive Ret activity in knock-in multiple endocrine neoplasia type B mice induces profound elevation of brain dopamine concentration via enhanced synthesis and increases the number of TH-positive cells in the substantia nigra

Ret is the common signaling receptor for glial cell line-derived neurotrophic factor (GDNF) and other ligands of the GDNF family that have potent effects on brain dopaminergic neurons. The Met918Thr mutation leads to constitutive activity of Ret receptor tyrosine kinase, causing the cancer syndrome called multiple endocrine neoplasia type B (MEN2B). We used knock-in MEN2B mice with the Ret-MEN2B mutation to study the effects of constitutive Ret activity on the brain dopaminergic system and found robustly increased concentrations of dopamine (DA) and its metabolites in the striatum, cortex, and hypothalamus. The concentrations of brain serotonin were not affected and those of noradrenaline were slightly increased only in the lower brainstem. Tyrosine hydroxylase (TH) protein levels were increased in the striatum and substantia nigra/ventral tegmental area (SN/VTA), and TH mRNA levels were increased in SN/VTA of MEN2B mice, suggesting that constitutive Ret activity increases DA levels by increasing its synthesis. Also, the striatal DA transporter protein levels in the MEN2B mice were increased, which agrees with increased sensitivity of these mice to the stimulatory effects of cocaine. In the SN pars compacta of homozygous MEN2B mice, we found a 26% increase in the number of TH-positive cells, but no differences were found in the VTA. Thus, we show here that the constitutive Ret activity in mice is sufficient to increase the number of dopaminergic neurons and leads to profound elevation of brain DA concentration. These data clearly suggest that Ret activity per se can have a direct biological function that actively changes and shapes the brain dopaminergic system.

Modulation of tyrosine hydroxylase expression by melatonin in human SH-SY5Y neuroblastoma cells

We have previously reported in vivo preservation of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis, following treatment with physiological doses of melatonin, in a 6-hydroxydopamine model of Parkinson's disease. Based on these findings, we postulated that melatonin would similarly modulate the expression of TH in vitro. Therefore, using human SH-SY5Y neuroblastoma cells which can differentiate into dopaminergic neurons following treatment with retinoic acid, we first examined whether these cells express melatonin receptors. Subsequently, the physiological dose-dependent effects of melatonin on TH expression were examined in both undifferentiated and differentiated cells. The novel detection of the G protein-coupled melatonin MT(1) receptor in SH-SY5Y cells by RT-PCR was confirmed by sequencing and Western blotting. In addition, following treatment of SH-SY5Y cells with melatonin (0.1-100 nM) for 24h, Western analysis revealed a significant increase in TH protein levels. A biphasic response, with significant increases in TH protein at 0.5 and 1 nM melatonin and a reversal at higher doses was seen in undifferentiated cells; whereas in differentiated cells, melatonin was effective at doses of 1 and 100 nM. These findings suggest a physiological role for melatonin in modulating TH expression, possibly via the MT(1) receptor.

Protein kinase C delta negatively regulates tyrosine hydroxylase activity and dopamine synthesis by enhancing protein phosphatase-2A activity in dopaminergic neurons

Tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis, can be regulated by phosphorylation at multiple serine residues, including serine-40. In the present study, we report a novel interaction between a key member of the novel PKC family, protein kinase Cdelta (PKCdelta), and TH, in which the kinase modulates dopamine synthesis by negatively regulating TH activity via protein phosphatase 2A (PP2A). We observed that PKCdelta is highly expressed in nigral dopaminergic neurons and colocalizes with TH. Interestingly, suppression of PKCdelta activity with the kinase inhibitor rottlerin, PKCdelta-small interfering RNA, or with PKCdelta dominant-negative mutant effectively increased a number of key biochemical events in the dopamine pathway, including TH-ser40 phosphorylation, TH enzymatic activity, and dopamine synthesis in neuronal cell culture models. Additionally, we found that PKCdelta not only physically associates with the PP2A catalytic subunit (PP2Ac) but also phosphorylates the phosphatase to increase its activity. Notably, inhibition of PKCdelta reduced the dephosphorylation activity of PP2A and thereby increased TH-ser40 phosphorylation, TH activity, and dopamine synthesis. To further validate our findings, we used the PKCdelta knock-out (PKCdelta-/-) mouse model. Consistent with other results, we found greater TH-ser40 phosphorylation and reduced PP2A activity in the substantia nigra of PKCdelta-/- mice than in wild-type mice. Importantly, this was accompanied by an increased dopamine level in the striatum of PKCdelta-/- mice. Collectively, these results suggest that PKCdelta phosphorylates PP2Ac to enhance its activity and thereby reduces TH-ser40 phosphorylation and TH activity and ultimately dopamine synthesis.

3-Hydroxymorphinan, a metabolite of dextromethorphan, protects nigrostriatal pathway against MPTP-elicited damage both in vivo and in vitro

We investigated the neuroprotective property of analogs of dextromethorphan (DM) in lipopolysaccharide (LPS) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) models to identify neuroprotective drugs for Parkinson's disease (PD). In vivo studies showed that daily injections with DM analogs protected dopamine (DA) neurons in substantia nigra pars compacta and restored DA levels in striatum using two different models for PD. Of the five analogs studied, 3-hydroxymorphinan (3-HM), a metabolite of DM, was the most potent, and restored DA neuronal loss and DA depletion up to 90% of the controls. Behavioral studies showed an excellent correlation between potency for preventing toxin-induced decrease in motor activities and neuroprotective effects among the DM analogs studied, of which 3-HM was the most potent in attenuating behavioral damage. In vitro studies revealed two glia-dependent mechanisms for the neuroprotection by 3-HM. First, astroglia mediated the 3-HM-induced neurotrophic effect by increasing the gene expression of neurotrophic factors, which was associated with the increased acetylation of histone H3. Second, microglia participated in 3-HM-mediated neuroprotection by reducing MPTP-elicited reactive microgliosis as evidenced by the decreased production of reactive oxygen species. In summary, we show the potent neuroprotection by 3-HM in LPS and MPTP PD models investigated. With its high efficacy and low toxicity, 3-HM may be a novel therapy for PD.

Interleukin-1beta mediates GDNF up-regulation upon dopaminergic injury in ventral midbrain cell cultures

We recently proposed the involvement of diffusible modulators in signalling astrocytes to increase glial cell line-derived neurotrophic factor (GDNF) expression after selective dopaminergic injury by H2O2 or L-DOPA. Here we report that interleukin-1beta (IL-1beta) is involved in this crosstalk between injured neurons and astrocytes. IL-1beta was detected only in the media from challenged neuron-glia cultures. Exogenous IL-1beta did not change GDNF protein levels in astrocyte cultures, and diminished GDNF levels in neuron-glia cultures. This decrease was not due to cell loss, as assessed by the MTT assay and immunocytochemistry. Neither H2O2 nor L-DOPA induced microglia proliferation or appeared to change its activation state. The IL-1 receptor antagonist (IL-1ra) prevented GDNF up-regulation in challenged cultures, showing that IL-1beta is involved in the signalling between injured neurons and astrocytes. Since IL-1ra decreased the number of dopaminergic neurons in H2O2-treated cultures, we propose that IL-1 has a neuroprotective role in this system involving GDNF up-regulation.

Autoregulation of glial cell line-derived neurotrophic factor expression: implications for the long-lasting actions of the anti-addiction drug, Ibogaine

We recently showed that the up-regulation of the glial cell line-derived neurotrophic factor (GDNF) pathway in the midbrain, is the molecular mechanism by which the putative anti-addiction drug Ibogaine mediates its desirable action of reducing alcohol consumption. Human reports and studies in rodents have shown that a single administration of Ibogaine results in a long-lasting reduction of drug craving (humans) and drug and alcohol intake (rodents). Here we determine whether, and how, Ibogaine exerts its long-lasting actions on GDNF expression and signaling. Using the dopaminergic-like SHSY5Y cell line as a culture model, we observed that short-term Ibogaine exposure results in a sustained increase in GDNF expression that is mediated via the induction of a long-lasting autoregulatory cycle by which GDNF positively regulates its own expression. We show that the initial exposure of cells to Ibogaine or GDNF results in an increase in GDNF mRNA, leading to protein expression and to the corresponding activation of the GDNF signaling pathway. This, in turn, leads to a further increase in the mRNA level of the growth factor. The identification of a GDNF-mediated, autoregulatory long-lasting feedback loop could have important implications for GDNF's potential value as a treatment for addiction and neurodegenerative diseases.

Glial cell-line derived neurotrophic factor and neurturin regulate the expressions of distinct miRNA precursors through the activation of GFRalpha2

Glial cell line-derived neurotrophic factor (GDNF) and neurturin (NTN) are structurally related neurotrophic factors that have both been shown to prevent the degeneration of dopaminergic neurons in vitro and in vivo. NTN and GDNF are thought to bind with different affinities to the GDNF family receptor alpha-2 (GFRalpha2), and can activate the same multi-component receptor system consisting of GFRalpha2, receptor tyrosine kinase Ret (RET) and NCAM. MicroRNAs (miRNAs) are a class of short, non-coding RNAs that regulate gene expression through translational repression or RNA degradation. miRNAs have diverse functions, including regulating differentiation, proliferation and apoptosis in several organisms. It is currently unknown whether GDNF and NTN regulate the expression of miRNAs through activation of the same multi-component receptor system. Using quantitative real-time PCR, we measured the expression of some miRNA precursors in human BE(2)-C cells that express GFRalpha2 but not GFRalpha1. GDNF and NTN differentially regulate the expression of distinct miRNA precursors through the activation of mitogen-activated protein kinase (extracellular signal-regulated kinase 1/2). This study showed that the expression of distinct miRNA precursors is differentially regulated by specific ligands through the activation of GFRalpha2.

Enhanced catecholamine synthesis in the prefrontal cortex after traumatic brain injury: implications for prefrontal dysfunction

Traumatic brain injury (TBI)--induced dysfunction of the prefrontal cortex causes many high-level cognitive deficits, including working memory (WM) dysfunction. WM lies at the core of many high-level functions, yet the cellular and molecular mechanisms underlying its dysfunction are poorly understood. Lesion and pharmacological studies in rodents have implicated the medial prefrontal cortex (mPFC), which includes the prelimbic/infralimbic (PL/IL) cortices, in WM tasks. These studies have shown that optimal levels of catecholamine neurotransmission are critical for normalcy of WM function, suggesting that alterations in their synthesis may play a role in WM dysfunction. Using the cortical impact injury model of traumatic brain injury which reproducibly causes working memory deficits in rodents, we have measured the protein levels and activity of tyrosine hydroxylase (TH), the rate-limiting enzyme for catecholamine biosynthesis, and tissue dopamine (DA) and norepinephrine (NE) levels in microdissected PL/IL tissues. Our results show that TBI increases TH protein levels, its activity and tissue DA and NE content in the PL/IL. These findings suggest that altered catecholamine signaling within the PL/IL may contribute to impaired PFC function, and may have implications in the design and implementation of strategies to alleviate prefrontal dysfunction in brain injury patients.

Enhancement of tyrosine hydroxylase expression and activity by Trypanosoma cruzi parasite-derived neurotrophic factor

A parasite-derived protein, PDNF, produced by the Chagas' disease agent Trypanosoma cruzi, functionally mimics mammalian neurotrophic factors by delaying apoptotic death and promoting survival and differentiation of neurons, including dopaminergic cells, through the activation of nerve growth factor receptor TrkA. Because it is well established that neurotrophic factors regulate enzymes involved in the biosynthesis of neurotransmitters, we examined whether PDNF could also directly activate tyrosine hydroxylase (TH), a rate-limiting enzyme in the synthesis of dopamine and other catecholamine neurotransmitters. We found that primary cultures of rat ventral mesencephalon responded to PDNF by increasing the number of TH-positive neurons and, most importantly, preserved expression of TH in neurons treated with Parkinson disease-inducing neurotoxin 1-methyl-4-phenyl pyridinium (MPP(+)). In dopaminergic PC12 cells, PDNF induced TH transcription via CRE element in TH promoter followed by significant increase in TH protein and expansion of TH-positive cell population. Furthermore, PDNF stimulated TH enzymatic activity by enhancing phosphorylation of seryl residues 31 and 40 through the activation of MAPK/Erk1/2 and cAMP-dependent protein kinase A signaling, respectively. Therefore, our results indicate that PDNF, in addition to its functioning as survival and differentiation-promoting factor for dopaminergic neuronal cells, can directly influence activity of the rate-limiting enzyme that underlies catecholamine biosynthetic cascade. This novel feature of PDNF should help understand the mechanism of neuronal function altered by T. cruzi infection, specifically neurotransmitter secretion. In addition, the findings have potential implications in the therapy of Chagas' and other neurodegenerative disorders.

GDNF abates serum deprivation-induced tyrosine hydroxylase Ser19 phosphorylation and activity

High dopamine levels can contribute to neuronal dysfunction, impair plasticity and be toxic to neuronal cells in pathological conditions. The synthesis of dopamine is regulated by phosphorylation of the rate-limiting enzyme tyrosine hydroxylase (TH) under physiological conditions, with the phosphorylation of Ser31 and Ser40 directly increasing TH activity. Although a third phosphorylation site, Ser19, does not appear to directly regulate TH activity in physiological conditions, its role in pathological conditions is poorly understood. In this study, we examined the effects of serum deprivation (to mimic loss of retrogradely/anterogradely transported target-derived neurotrophic factors following axonal injury) and glutamate receptor stimulation (to mimic excitotoxicity) on TH phosphorylation and activity in a cell line and in mesencephalic primary culture cells. In addition, we also tested whether glial cell line-derived neurotrophic factor (GDNF) can alter these changes. We demonstrate that serum-deprivation resulted in a sustained increase in Ser19 phosphorylation beginning at 3 h and lasting up to 10 h without any detectable change in Ser31 or Ser40 phosphorylation within this time frame. This increase in Ser19 phosphorylation was associated with enhanced TH activity and was due, in part, to glutamate-receptor-mediated calcium influx and possibly calcium/calmodulin-dependent protein kinase II (CaMKII) activation. Interestingly in this serum-deprivation model, GDNF blocked the increase in Ser19 phosphorylation and TH activity at the 10-h time point following serum deprivation. Furthermore, GDNF also blocked the glutamate-mediated increase in Ser19 phosphorylation in rat primary mesencephalic neuronal cultures. Taken together, these findings suggest that GDNF may reduce dopamine synthesis in pathological conditions.

Plasticity of pelvic autonomic ganglia and urogenital innervation

Pelvic ganglia contain a mixture of sympathetic and parasympathetic neurons and provide most of the motor innervation of the urogenital organs. They show a remarkable sensitivity to androgens and estrogens, which impacts on their development into sexually dimorphic structures and provide an array of mechanisms by which plasticity of these neurons can occur during puberty and adulthood. The structure of pelvic ganglia varies widely among species, ranging from rodents, which have a pair of large ganglia, to humans, in whom pelvic ganglion neurons are distributed in a large, complex plexus. This plexus is frequently injured during pelvic surgical procedures, yet strategies for its repair have yet to be developed. Advances in this area will come from a better understanding of the effects of injury on the cellular signaling process in pelvic neurons and also the role of neurotrophic factors during development, maintenance, and repair of these axons.

Lentiviral nigral delivery of GDNF does not prevent neurodegeneration in a genetic rat model of Parkinson's disease

Viral delivery of glial cell line-derived neurotrophic factor (GDNF) currently represents one of the most promising neuroprotective strategies for Parkinson's Disease (PD). However, the effect of this neurotrophic factor has never been tested in the newly available genetic models of PD based on the viral expression of mutated alpha-synuclein. In this study, we evaluated the ability of lentiviral vectors coding for GDNF (lenti-GDNF) to prevent nigral dopaminergic degeneration associated with the lentiviral mediated expression of the A30P mutant human alpha-synuclein (lenti-A30P). This virally based rat model develops a progressive and selective loss of dopamine neurons associated with the appearance of alpha-synuclein containing inclusions, thus recapitulating the major hallmarks of PD. Lenti-GDNF was injected in the substantia nigra 2 weeks before nigral administration of lenti-A30P. Although a robust expression of GDNF was observed in the whole nigrostriatal pathway due to retrograde and/or anterograde transport, lenti-GDNF did not prevent the alpha-synuclein-induced dopaminergic neurodegeneration in the lentiviral-based genetic rat model of PD. These results suggest that sustained GDNF treatment cannot modulate the cellular toxicity related to abnormal folded protein accumulation as mutated human alpha-synuclein.

Cyclin-dependent Kinase 5 Phosphorylates Serine 31 of Tyrosine Hydroxylase and Regulates Its Stability

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine biosynthesis, and its activity is regulated by phosphorylation in the N-terminal regulatory domain. The proline-directed serine/threonine kinase cyclin-dependent kinase 5 (cdk5) plays an important role in diverse neuronal processes. In the present study, we identify TH as a novel substrate of cdk5. We show that cdk5 phosphorylates TH at serine 31 and that this phosphorylation is associated with an increase in total TH activity. In transgenic mice with increased cdk5 activity, the immunoreactivity for phosphorylated TH at Ser-31 is enhanced in neurons of the substantia nigra, a brain region enriched with TH-positive neurons. In addition, we demonstrate that co-expression of cdk5 and its regulatory activator p35 with TH increases the stability of TH. Consistent with these findings, TH protein levels are reduced in cdk5 knock-out mice. Importantly, the TH activity and protein turnover of the phosphorylation-defective mutant TH S31A was not altered by cdk5 activity. Taken together, these data suggest that cdk5 phosphorylation of TH is an important regulator of TH activity through stabilization of TH protein levels.

Tyrosine hydroxylase phosphorylation: regulation and consequences

The rate-limiting enzyme in catecholamine synthesis is tyrosine hydroxylase. It is phosphorylated at serine (Ser) residues Ser8, Ser19, Ser31 and Ser40 in vitro, in situ and in vivo. A range of protein kinases and protein phosphatases are able to phosphorylate or dephosphorylate these sites in vitro. Some of these enzymes are able to regulate tyrosine hydroxylase phosphorylation in situ and in vivo but the identity of the kinases and phosphatases is incomplete, especially for physiologically relevant stimuli. The stoichiometry of tyrosine hydroxylase phosphorylation in situ and in vivo is low. The phosphorylation of tyrosine hydroxylase at Ser40 increases the enzyme's activity in vitro, in situ and in vivo. Phosphorylation at Ser31 also increases the activity but to a much lesser extent than for Ser40 phosphorylation. The phosphorylation of tyrosine hydroxylase at Ser19 or Ser8 has no direct effect on tyrosine hydroxylase activity. Hierarchical phosphorylation of tyrosine hydroxylase occurs both in vitro and in situ, whereby the phosphorylation at Ser19 increases the rate of Ser40 phosphorylation leading to an increase in enzyme activity. Hierarchical phosphorylation depends on the state of the substrate providing a novel form of control of tyrosine hydroxylase activation.