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Absence of Ret signaling in mice causes progressive and late degeneration of the nigrostriatal system.

Kramer ER, Aron L, Ramakers GM, Seitz S, Zhuang X, Beyer K, Smidt MP, Klein R - PLoS Biol. (2007)

Bottom Line: Support of ageing neurons by endogenous neurotrophic factors such as glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) may determine whether the neurons resist or succumb to neurodegeneration.We find that Ret, but not TrkB, ablation causes progressive and adult-onset loss of DA neurons specifically in the substantia nigra pars compacta, degeneration of DA nerve terminals in striatum, and pronounced glial activation.These findings establish Ret as a critical regulator of long-term maintenance of the nigrostriatal DA system and suggest conditional Ret mutants as useful tools for gaining insights into the molecular mechanisms involved in the development of PD.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Neurobiology, Max-Planck Institute of Neurobiology, Martinsried, Germany. ekramer@neuro.mpg.de

ABSTRACT
Support of ageing neurons by endogenous neurotrophic factors such as glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) may determine whether the neurons resist or succumb to neurodegeneration. GDNF has been tested in clinical trials for the treatment of Parkinson disease (PD), a common neurodegenerative disorder characterized by the loss of midbrain dopaminergic (DA) neurons. BDNF modulates nigrostriatal functions and rescues DA neurons in PD animal models. The physiological roles of GDNF and BDNF signaling in the adult nigrostriatal DA system are unknown. We generated mice with regionally selective ablations of the genes encoding the receptors for GDNF (Ret) and BDNF (TrkB). We find that Ret, but not TrkB, ablation causes progressive and adult-onset loss of DA neurons specifically in the substantia nigra pars compacta, degeneration of DA nerve terminals in striatum, and pronounced glial activation. These findings establish Ret as a critical regulator of long-term maintenance of the nigrostriatal DA system and suggest conditional Ret mutants as useful tools for gaining insights into the molecular mechanisms involved in the development of PD.

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Reduced Dopamine Release in the Striatum of DAT-Retlx/lx Mice(A) Total dopamine levels normalized to 2,3-dihydroxybenzoic acid (DHBA) and expressed relative to the weight of wet striatum (grams) of 2-y-old control mice (Retlx/lx), heterozygous Retlx/−, heterozygous DAT-Retlx/+, homozygous DAT-Retlx/lx, and DAT-TrkBlx/lx mice. Note the minor reduction of total dopamine levels in all mice carrying the DAT-Cre knock-in construct.(B–E) Evoked dopamine release after electrical stimulation in the dorsal striatum of control mice (Retlx/lx and Retlx/− mice), heterozygous DAT-Retlx/+ mice, and homozygous DAT-Retlx/lx mice of 1 y (B and C) or 2 y (D and E) of age. In both age groups, there is a significant decrease of released dopamine in the mice carrying the DAT-Cre knock-in construct compared to controls. There is a further significant decrease in the homozygous DAT-Retlx/lx mice due to the lack of Ret (n = 5 per genotype, p < 0.05, Student t-test). *, p < 0.05; **, p < 0.01 (Student t-test). (C and E) Representative traces of single evoked dopamine release in different control and mutant mice.
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pbio-0050039-g007: Reduced Dopamine Release in the Striatum of DAT-Retlx/lx Mice(A) Total dopamine levels normalized to 2,3-dihydroxybenzoic acid (DHBA) and expressed relative to the weight of wet striatum (grams) of 2-y-old control mice (Retlx/lx), heterozygous Retlx/−, heterozygous DAT-Retlx/+, homozygous DAT-Retlx/lx, and DAT-TrkBlx/lx mice. Note the minor reduction of total dopamine levels in all mice carrying the DAT-Cre knock-in construct.(B–E) Evoked dopamine release after electrical stimulation in the dorsal striatum of control mice (Retlx/lx and Retlx/− mice), heterozygous DAT-Retlx/+ mice, and homozygous DAT-Retlx/lx mice of 1 y (B and C) or 2 y (D and E) of age. In both age groups, there is a significant decrease of released dopamine in the mice carrying the DAT-Cre knock-in construct compared to controls. There is a further significant decrease in the homozygous DAT-Retlx/lx mice due to the lack of Ret (n = 5 per genotype, p < 0.05, Student t-test). *, p < 0.05; **, p < 0.01 (Student t-test). (C and E) Representative traces of single evoked dopamine release in different control and mutant mice.

Mentions: PD is clinically defined by a decrease in dopamine levels that result in motor impairments. To determine the effects of nerve terminal loss in DAT-Retlx/lx mice on the DA output capacity of the system, we measured total levels and evoked dopamine release in the striatum of mutant mice. Striatal levels of dopamine and one of its major metabolites, dihydroxyphenylacetic acid (DOPAC), were similar in DAT-Retlx/lx and control mice at 3, 12, and 24 mo (Figure 7A and unpublished data). The somewhat lower values in all mice (mutants and controls) carrying the DAT-Cre transgene compared to DAT-Cre–negative controls is due to the reduced levels of DAT protein, which regulates dopamine transport and metabolism [31]. Evoked dopamine release was measured by fast-scan cyclic voltammetry (FSCV) following electrical stimulation of coronal slice preparations of mutant and control mice [32]. Electrical stimulation resulted in a stimulus intensity–dependent overflow of DA in the striatum. DA overflow is the result of released DA minus the DA reuptake by DAT. We observed a marked reduction of evoked DA overflow in the striatum of all mice carrying the DAT-Cre transgene (Figure 7B–7E) as described before [31]. Interestingly, the evoked DA overflow was further reduced in DAT-Retlx/lx mice compared to DAT-Retlx/+control mice in both 1-y-old and 2-y-old mutants (n = 5 per genotype, p < 0.05 and p < 0.01 for 1-y-old and 2-y-old mutants, respectively, one-way analysis of variance and post-hoc Student t-test). Together with the unchanged input–output curves of the FSCV experiment (Figure S4), these data suggest that the reduced dopamine release and reuptake in the Ret mutants is likely due to the reduced number of DA fibers in the striatum. To determine to what extent these histological and physiological alterations change the behavior of the DAT-Retlx/lx mice, we tested DAT-Retlx/lx and control mice for behavioral alterations in open-field and rotarod tasks, and in voluntary and forced swimming tasks (Protocol S1). The behavior was essentially unaffected in the mutants (Figure S5).


Absence of Ret signaling in mice causes progressive and late degeneration of the nigrostriatal system.

Kramer ER, Aron L, Ramakers GM, Seitz S, Zhuang X, Beyer K, Smidt MP, Klein R - PLoS Biol. (2007)

Reduced Dopamine Release in the Striatum of DAT-Retlx/lx Mice(A) Total dopamine levels normalized to 2,3-dihydroxybenzoic acid (DHBA) and expressed relative to the weight of wet striatum (grams) of 2-y-old control mice (Retlx/lx), heterozygous Retlx/−, heterozygous DAT-Retlx/+, homozygous DAT-Retlx/lx, and DAT-TrkBlx/lx mice. Note the minor reduction of total dopamine levels in all mice carrying the DAT-Cre knock-in construct.(B–E) Evoked dopamine release after electrical stimulation in the dorsal striatum of control mice (Retlx/lx and Retlx/− mice), heterozygous DAT-Retlx/+ mice, and homozygous DAT-Retlx/lx mice of 1 y (B and C) or 2 y (D and E) of age. In both age groups, there is a significant decrease of released dopamine in the mice carrying the DAT-Cre knock-in construct compared to controls. There is a further significant decrease in the homozygous DAT-Retlx/lx mice due to the lack of Ret (n = 5 per genotype, p < 0.05, Student t-test). *, p < 0.05; **, p < 0.01 (Student t-test). (C and E) Representative traces of single evoked dopamine release in different control and mutant mice.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC1808500&req=5

pbio-0050039-g007: Reduced Dopamine Release in the Striatum of DAT-Retlx/lx Mice(A) Total dopamine levels normalized to 2,3-dihydroxybenzoic acid (DHBA) and expressed relative to the weight of wet striatum (grams) of 2-y-old control mice (Retlx/lx), heterozygous Retlx/−, heterozygous DAT-Retlx/+, homozygous DAT-Retlx/lx, and DAT-TrkBlx/lx mice. Note the minor reduction of total dopamine levels in all mice carrying the DAT-Cre knock-in construct.(B–E) Evoked dopamine release after electrical stimulation in the dorsal striatum of control mice (Retlx/lx and Retlx/− mice), heterozygous DAT-Retlx/+ mice, and homozygous DAT-Retlx/lx mice of 1 y (B and C) or 2 y (D and E) of age. In both age groups, there is a significant decrease of released dopamine in the mice carrying the DAT-Cre knock-in construct compared to controls. There is a further significant decrease in the homozygous DAT-Retlx/lx mice due to the lack of Ret (n = 5 per genotype, p < 0.05, Student t-test). *, p < 0.05; **, p < 0.01 (Student t-test). (C and E) Representative traces of single evoked dopamine release in different control and mutant mice.
Mentions: PD is clinically defined by a decrease in dopamine levels that result in motor impairments. To determine the effects of nerve terminal loss in DAT-Retlx/lx mice on the DA output capacity of the system, we measured total levels and evoked dopamine release in the striatum of mutant mice. Striatal levels of dopamine and one of its major metabolites, dihydroxyphenylacetic acid (DOPAC), were similar in DAT-Retlx/lx and control mice at 3, 12, and 24 mo (Figure 7A and unpublished data). The somewhat lower values in all mice (mutants and controls) carrying the DAT-Cre transgene compared to DAT-Cre–negative controls is due to the reduced levels of DAT protein, which regulates dopamine transport and metabolism [31]. Evoked dopamine release was measured by fast-scan cyclic voltammetry (FSCV) following electrical stimulation of coronal slice preparations of mutant and control mice [32]. Electrical stimulation resulted in a stimulus intensity–dependent overflow of DA in the striatum. DA overflow is the result of released DA minus the DA reuptake by DAT. We observed a marked reduction of evoked DA overflow in the striatum of all mice carrying the DAT-Cre transgene (Figure 7B–7E) as described before [31]. Interestingly, the evoked DA overflow was further reduced in DAT-Retlx/lx mice compared to DAT-Retlx/+control mice in both 1-y-old and 2-y-old mutants (n = 5 per genotype, p < 0.05 and p < 0.01 for 1-y-old and 2-y-old mutants, respectively, one-way analysis of variance and post-hoc Student t-test). Together with the unchanged input–output curves of the FSCV experiment (Figure S4), these data suggest that the reduced dopamine release and reuptake in the Ret mutants is likely due to the reduced number of DA fibers in the striatum. To determine to what extent these histological and physiological alterations change the behavior of the DAT-Retlx/lx mice, we tested DAT-Retlx/lx and control mice for behavioral alterations in open-field and rotarod tasks, and in voluntary and forced swimming tasks (Protocol S1). The behavior was essentially unaffected in the mutants (Figure S5).

Bottom Line: Support of ageing neurons by endogenous neurotrophic factors such as glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) may determine whether the neurons resist or succumb to neurodegeneration.We find that Ret, but not TrkB, ablation causes progressive and adult-onset loss of DA neurons specifically in the substantia nigra pars compacta, degeneration of DA nerve terminals in striatum, and pronounced glial activation.These findings establish Ret as a critical regulator of long-term maintenance of the nigrostriatal DA system and suggest conditional Ret mutants as useful tools for gaining insights into the molecular mechanisms involved in the development of PD.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular Neurobiology, Max-Planck Institute of Neurobiology, Martinsried, Germany. ekramer@neuro.mpg.de

ABSTRACT
Support of ageing neurons by endogenous neurotrophic factors such as glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) may determine whether the neurons resist or succumb to neurodegeneration. GDNF has been tested in clinical trials for the treatment of Parkinson disease (PD), a common neurodegenerative disorder characterized by the loss of midbrain dopaminergic (DA) neurons. BDNF modulates nigrostriatal functions and rescues DA neurons in PD animal models. The physiological roles of GDNF and BDNF signaling in the adult nigrostriatal DA system are unknown. We generated mice with regionally selective ablations of the genes encoding the receptors for GDNF (Ret) and BDNF (TrkB). We find that Ret, but not TrkB, ablation causes progressive and adult-onset loss of DA neurons specifically in the substantia nigra pars compacta, degeneration of DA nerve terminals in striatum, and pronounced glial activation. These findings establish Ret as a critical regulator of long-term maintenance of the nigrostriatal DA system and suggest conditional Ret mutants as useful tools for gaining insights into the molecular mechanisms involved in the development of PD.

Show MeSH
Related in: MedlinePlus