Parkin cooperates with GDNF/RET signaling to prevent dopaminergic neuron degeneration

Parkin and the glial cell line-derived neurotrophic factor (GDNF) receptor RET have both been independently linked to the dopaminergic neuron degeneration that underlies Parkinson's disease (PD). In the present study, we demonstrate that there is genetic crosstalk between parkin and the receptor tyrosine kinase RET in two different mouse models of PD. Mice lacking both parkin and RET exhibited accelerated dopaminergic cell and axonal loss compared with parkin-deficient animals, which showed none, and RET-deficient mice, in which we found moderate degeneration. Transgenic expression of parkin protected the dopaminergic systems of aged RET-deficient mice. Downregulation of either parkin or RET in neuronal cells impaired mitochondrial function and morphology. Parkin expression restored mitochondrial function in GDNF/RET-deficient cells, while GDNF stimulation rescued mitochondrial defects in parkin-deficient cells. In both cases, improved mitochondrial function was the result of activation of the prosurvival NF-κB pathway, which was mediated by RET through the phosphoinositide-3-kinase (PI3K) pathway. Taken together, these observations indicate that parkin and the RET signaling cascade converge to control mitochondrial integrity and thereby properly maintain substantia nigra pars compacta dopaminergic neurons and their innervation in the striatum. The demonstration of crosstalk between parkin and RET highlights the interplay in the protein network that is altered in PD and suggests potential therapeutic targets and strategies to treat PD.

Glial cell-line derived neurotrophic factor (GDNF) replacement attenuates motor impairments and nigrostriatal dopamine deficits in 12-month-old mice with a partial deletion of GDNF

Glial cell-line derived neurotrophic factor (GDNF) has been established as a growth factor for the survival and maintenance of dopamine (DA) neurons. In phase I clinical trials, GDNF treatment in Parkinson's disease patients led to improved motor function and GDNF has been found to be down regulated in Parkinson's disease patients. Studies using GDNF heterozygous (Gdnf(+/-)) mice have demonstrated that a partial reduction of GDNF leads to an age-related accelerated decline in nigrostriatal DA system- and motor-function and increased neuro-inflammation and oxidative stress in the substantia nigra (SN). Therefore, the purpose of the current studies was to determine if GDNF replacement restores motor function and functional markers within the nigrostriatal DA system in middle-aged Gdnf(+/-) mice. At 11months of age, male Gdnf(+/-) and wildtype (WT) mice underwent bilateral intra-striatal injections of GDNF (10μg) or vehicle. Locomotor activity was assessed weekly 1-4weeks after treatment. Four weeks after treatment, their brains were processed for analysis of GDNF levels and various DAergic and oxidative stress markers. An intrastriatal injection of GDNF increased motor activity in Gdnf(+/-) mice to levels comparable to WT mice (1week after injection) and this effect was maintained through the 4-week time point. This increase in locomotion was accompanied by a 40% increase in striatal GDNF protein levels and SN GDNF expression in Gdnf(+/-) mice. Additionally, GDNF treatment significantly increased the number of tyrosine hydroxylase (TH)-positive neurons in the SN of middle-aged Gdnf(+/-) mice, but not WT mice, which was coupled with reduced oxidative stress in the SN. These studies further support that long-term changes related to the dysfunction of the nigrostriatal pathway are influenced by GDNF expression and add that this dysfunction appears to be responsive to GDNF treatment. Additionally, these studies suggest that long-term GDNF depletion alters the biological and behavioral responses to GDNF treatment.

Vesico-ureteric reflux and urinary tract development in the Pax2 1Neu+/- mouse

Vesico-ureteric reflux (VUR) is a urinary tract abnormality that affects roughly one-third of patients with renal-coloboma syndrome, an autosomal dominant condition caused by a mutation in PAX2. Here, we report that a mouse model with an identical mutation, the Pax2 1Neu+/- mouse, has a 30% incidence of VUR. In VUR, urine flows retrogradely from the bladder to the ureter and is associated with urinary tract infections, hypertension, and renal failure. The propensity to reflux in the Pax2 1Neu+/- mouse is correlated with a shortened intravesical ureter that has lost its oblique angle of entry into the bladder wall compared with wild-type mice. Normally, the kidney and urinary tract develop from the ureteric bud, which grows from a predetermined position on the mesonephric duct. In Pax2 1Neu+/- mice, this position is shifted caudally while surrounding metanephric mesenchyme markers remain unaffected. Mutant offspring from crosses between Pax2 1Neu+/- and Hoxb7/GFP+/- mice have delayed union of the ureter with the bladder and delayed separation of the ureter from the mesonephric duct. These events are not caused by a change in apoptosis within the developing urinary tract. Our results provide the first evidence that VUR may arise from a delay in urinary tract maturation and an explanation for the clinical observation that VUR resolves over time in some affected children.

Long-term consequences of methamphetamine exposure in young adults are exacerbated in glial cell line-derived neurotrophic factor heterozygous mice

Methamphetamine abuse in young adults has long-term deleterious effects on brain function that are associated with damage to monoaminergic neurons. Administration of glial cell line-derived neurotrophic factor (GDNF) protects dopamine neurons from the toxic effects of methamphetamine in animal models. Therefore, we hypothesized that a partial GDNF gene deletion would increase the susceptibility of mice to methamphetamine neurotoxicity during young adulthood and possibly increase age-related deterioration of behavior and dopamine function. Two weeks after a methamphetamine binge (4 x 10 mg/kg, i.p., at 2 h intervals), GDNF(+/-) mice had a significantly greater reduction of tyrosine hydroxylase immunoreactivity in the medial striatum, a proportionally greater depletion of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the striatum, and a greater increase in activated microglia in the substantia nigra than wild-type mice. At 12 months of age, methamphetamine-treated GDNF(+/-) mice exhibited less motor activity and lower levels of tyrosine hydroxylase-immunoreactivity, dopamine, DOPAC, and serotonin than wild-type mice. Greater striatal dopamine transporter activity in GDNF(+/-) mice may underlie their differential response to methamphetamine. These data suggest the possibility that methamphetamine use in young adults, when combined with lower levels of GDNF throughout life, may precipitate the appearance of parkinsonian-like behaviors during aging.

An inducer for glial cell line-derived neurotrophic factor and tumor necrosis factor-alpha protects against methamphetamine-induced rewarding effects and sensitization

BACKGROUND

There are few efficacious medications for drug dependence. We investigated the potential of Leu-Ile, which induces the expression of glial cell line-derived neurotrophic factor (GDNF) and tumor necrosis factor-alpha (TNF-alpha), as a novel therapeutic agent for methamphetamine (METH)-induced dependence.

METHODS

The levels of GDNF and TNF-alpha messenger RNA (mRNA) were determined by real-time reverse transcription polymerase chain reaction. Enzyme immunoassays and immunohistochemistry were employed to determine levels of these proteins. Effects of Leu-Ile on METH-induced rewarding effects and sensitization were investigated with conditioned place preference and locomotor activity tests. Extracellular dopamine (DA) levels and DA uptake into synaptosomes were examined with an in vivo microdialysis and trititated thymidine ([(3)H]) DA uptake assay.

RESULTS

Leu-Ile induced the expression of not only GDNF but also TNF-alpha. Pretreatment with Leu-Ile blocked the acquisition of METH-induced place preference and sensitization. Interestingly, post-treatment with Leu-Ile attenuated them even after their development. An inhibitory effect of Leu-Ile on METH-induced place preference was observed in neither GDNF heterozygous nor TNF-alpha knockout mice. Leu-Ile inhibited DA release in the nucleus accumbens and the decrease in synaptosomal DA uptake in the midbrain induced by repeated METH treatment.

CONCLUSIONS

These results suggest that Leu-Ile inhibits METH-induced rewarding effects and sensitization by regulating extracellular DA levels via the induction of GDNF and TNF-alpha expression.

Enduring vulnerability to reinstatement of methamphetamine‐seeking behavior in glial cell line‐derived neurotrophic factor mutant mice

Genetic factors are considered to play an important role in drug dependence/addiction including the development of drug dependence and relapse. With the use of a model of drug self-administration in mutant mice, several specific genes and proteins have been identified as potentially important in the development of drug dependence. In contrast, little is known about the role of specific genes in enduring vulnerability to relapse, a clinical hallmark of drug addiction. Using a mouse model of reinstatement, which models relapse of drug-seeking behavior in addicts, we provide evidence that a partial reduction in the expression of the glial cell line-derived neurotrophic factor (GDNF) potentiates methamphetamine (METH) self-administration, enhances motivation to take METH, increases vulnerability to drug-primed reinstatement, and prolongs cue-induced reinstatement of extinguished METH-seeking behavior. In contrast, there was no significant difference in novelty responses, METH-stimulated hyperlocomotion and locomotor sensitization, food-reinforced operant behavior and motivation, or reinstatement of food-seeking behavior between GDNF heterozygous knockout mice and wild-type littermates. These findings suggest that GDNF may be associated with enduring vulnerability to reinstatement of METH-seeking behavior and a potential target in the development of therapies to control relapse.

A partial GDNF depletion leads to earlier age-related deterioration of motor function and tyrosine hydroxylase expression in the substantia nigra

Glial cell line-derived neurotrophic factor (GDNF) is a trophic factor for peripheral organs, spinal cord, and midbrain dopamine (DA) neurons. Levels of GDNF deteriorate in the substantia nigra in Parkinson's disease (PD). A heterozygous mouse model was created to assess whether chronic reductions in this neurotrophic factor impact motor function and the nigrostriatal dopamine system during the aging process. Due to the important role GDNF plays in kidney development, kidney function and histology were assessed and were found to be normal in both wild-type (WT) and GDNF+/- mice up to 22 months of age. Further, the animals of both genotypes had similar weights throughout the experiment. Locomotor activity was assessed for male WT and GDNF+/- mice at 4-month intervals from 4 to 20 months of age. Both GDNF+/- and WT mice exhibited an age-related decline in horizontal activity, although this was found 4 months earlier in GDNF+/- mice, at 12 months of age. Comparison of young (8 month old) and aged (20 month old) GDNF+/- and WT mice on an accelerating rotarod apparatus established a deficiency for aged but not young GDNF+/- mice, while aged WT mice performed as well as young WT mice on this task. Finally, both WT and GDNF+/- mice exhibited an age-related decrease in substantia nigra TH immunostaining, which was accelerated in the GDNF+/- mice. These behavioral and histological alterations suggest that GDNF may be an important factor for maintenance of motor coordination and spontaneous activity as well as DA neuronal function during aging, and further suggest that GDNF+/- mice may serve as a model for neuroprotective or rescue studies.

In heterozygous GDNF knockout mice the response of striatal dopaminergic system to acute morphine is altered

Glial cell line-derived neurotrophic factor (GDNF) regulates striatal dopaminergic neurons. To study whether reduced endogenous GDNF affect morphine's effects on striatal dopamine transmission, we estimated extracellular concentrations of dopamine and its metabolites by microdialysis in vivo and tissue concentrations post mortem in mice lacking one GDNF allele (GDNF+/- mice). In the wild-type mice, acute morphine (5 and 10 mg/kg s.c.) increased accumbal dopamine output dose-dependently (maximally by 30 and 80%, respectively). In the GDNF+/- mice, 5 mg/kg of morphine enhanced the accumbal dopamine output maximally by 110%, and significantly more than morphine 10 mg/kg (maximally by 60%). Also, the response of extracellular accumbal DOPAC to acute morphine was significantly altered in the GDNF+/- mice. In mice of both genotypes, the responses to morphine in the caudate putamen were similar to but much less intense than those in the nucleus accumbens. Morphine at the doses 5, 10, and 30 mg/kg dose-dependently elevated the striatal tissue concentrations of DOPAC and HVA, but the effect of 30 mg/kg was significantly smaller in the GDNF+/- mice than in their wild-type littermates. The binding of [(3)H]DAMGO to striatal membrane homogenates was similar between the genotypes. However, morphine induced antinociception in the GDNF+/- mice at a smaller dose than in the controls. The finding that reduced GDNF level alters the effects of morphine on striatal dopamine and our previous findings of elevated extracellular striatal dopamine concentrations and FosB/DeltaFosB expression in the GDNF+/- mice show the importance of GDNF in the regulation of striatal dopaminergic system.