Injury and strain-dependent dopaminergic neuronal degeneration in the substantia nigra of mice after axotomy or MPTP

In vivo functional diversity of midbrain dopamine neurons within identified axonal projections

Functional diversity of midbrain dopamine (DA) neurons ranges across multiple scales, from differences in intrinsic properties and connectivity to selective task engagement in behaving animals. Distinct in vitro biophysical features of DA neurons have been associated with different axonal projection targets. However, it is unknown how this translates to different firing patterns of projection-defined DA subpopulations in the intact brain. We combined retrograde tracing with single-unit recording and labelling in mouse brain to create an in vivo functional topography of the midbrain DA system. We identified differences in burst firing among DA neurons projecting to dorsolateral striatum. Bursting also differentiated DA neurons in the medial substantia nigra (SN) projecting either to dorsal or ventral striatum. We found differences in mean firing rates and pause durations among ventral tegmental area (VTA) DA neurons projecting to lateral or medial shell of nucleus accumbens. Our data establishes a high-resolution functional in vivo landscape of midbrain DA neurons.

The novel compound PBT434 prevents iron mediated neurodegeneration and alpha-synuclein toxicity in multiple models of Parkinson's disease

Elevated iron in the SNpc may play a key role in Parkinson's disease (PD) neurodegeneration since drug candidates with high iron affinity rescue PD animal models, and one candidate, deferirpone, has shown efficacy recently in a phase two clinical trial. However, strong iron chelators may perturb essential iron metabolism, and it is not yet known whether the damage associated with iron is mediated by a tightly bound (eg ferritin) or lower-affinity, labile, iron pool. Here we report the preclinical characterization of PBT434, a novel quinazolinone compound bearing a moderate affinity metal-binding motif, which is in development for Parkinsonian conditions. In vitro, PBT434 was far less potent than deferiprone or deferoxamine at lowering cellular iron levels, yet was found to inhibit iron-mediated redox activity and iron-mediated aggregation of α-synuclein, a protein that aggregates in the neuropathology. In vivo, PBT434 did not deplete tissue iron stores in normal rodents, yet prevented loss of substantia nigra pars compacta neurons (SNpc), lowered nigral α-synuclein accumulation, and rescued motor performance in mice exposed to the Parkinsonian toxins 6-OHDA and MPTP, and in a transgenic animal model (hA53T α-synuclein) of PD. These improvements were associated with reduced markers of oxidative damage, and increased levels of ferroportin (an iron exporter) and DJ-1. We conclude that compounds designed to target a pool of pathological iron that is not held in high-affinity complexes in the tissue can maintain the survival of SNpc neurons and could be disease-modifying in PD.

Inefficient DNA Repair Is an Aging-Related Modifier of Parkinson's Disease

The underlying relation between Parkinson’s disease (PD) etiopathology and its major risk factor, aging, is largely unknown. In light of the causative link between genome stability and aging, we investigate a possible nexus between DNA damage accumulation, aging, and PD by assessing aging-related DNA repair pathways in laboratory animal models and humans. We demonstrate that dermal fibroblasts from PD patients display flawed nucleotide excision repair (NER) capacity and that Ercc1 mutant mice with mildly compromised NER exhibit typical PD-like pathological alterations, including decreased striatal dopaminergic innervation, increased phospho-synuclein levels, and defects in mitochondrial respiration. Ercc1 mouse mutants are also more sensitive to the prototypical PD toxin MPTP, and their transcriptomic landscape shares important similarities with that of PD patients. Our results demonstrate that specific defects in DNA repair impact the dopaminergic system and are associated with human PD pathology and might therefore constitute an age-related risk factor for PD.

•

Ercc1-mediated DNA repair is necessary for preservation of dopaminergic neurons

•

Mouse mutants with mild Ercc1 defects display signs of dopaminergic pathology

•

Mild Ercc1 dysfunction is sensitized to the prototypical PD neurotoxin MPTP

•

PD patients’ peripheral cells exhibit inefficient nucleotide excision repair

Down-regulation of microglial activity attenuates axotomized nigral dopaminergic neuronal cell loss

BACKGROUND

There is growing evidence that inflammatory processes of activated microglia could play an important role in the progression of nerve cell damage in neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease which harbor features of chronic microglial activation, though the precise mechanism is unknown. In this study, we presented in vivo and ex vivo experimental evidences indicating that activated microglia could exacerbate the survival of axotomized dopaminergic neurons and that appropriate inactivation of microglia could be neuroprotective.

RESULTS

The transection of medial forebrain bundle (MFB) of a rat induced loss of dopaminergic neurons in a time-dependent manner and accompanied with microglial activation. Along with microglial activation, production of reactive oxygen species (ROS) was upregulated and TH/OX6/hydroethidine triple-immunofluorescence showed that the microglia mainly produced ROS. When the activated microglial cells that were isolated from the substantia nigra of the MFB axotomized animal, were transplanted into the substantia nigra of which MFB had been transected at 7 days ago, the survival rate of axotomized dopaminergic neurons was significantly reduced as compared with sham control. Meanwhile, when the microglial activation was attenuated by administration of tuftsin fragment 1-3 (microglia inhibitory factor) into the lateral ventricle using mini-osmotic pump, the survival rate of axotomized dopaminergic neurons was increased.

CONCLUSION

The present study suggests that activated microglia could actively produce and secrete unfavorable toxic substances, such as ROS, which could accelerate dopaminergic neuronal cell loss. So, well-controlled blockade of microglial activation might be neuroprotective in some neuropathological conditions.

Kainic acid-induced neuronal degeneration in hippocampal pyramidal neurons is driven by both intrinsic and extrinsic factors: analysis of FVB/N↔C57BL/6 chimeras

The excitotoxic effects of kainic acid (KA) in the mouse hippocampus is strain dependent. Following KA administration, the large majority of hippocampal pyramidal cells die in the FVB/N (FVB) mouse, while the pyramidal cells of the C57BL/6 (B6) strain are largely spared. We generated aggregation chimeras between the sensitive FVB and the resistant B6 strains to investigate whether intrinsic or extrinsic features of a neuron confer cell vulnerability or resistance to KA. The constitutive expression of transgenic green fluorescence protein (GFP) or β-galactosidase expressed from the ROSA26 locus was used to mark cells in FVB or B6 mice, respectively. These makers enable the identification of cells from each parental genotype while TUNEL (terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end labeling)-staining labeled dying cells. The analysis of the percentage of dying cells in FVB-GFP ↔ B6-ROSA chimeras yielded an intriguing mix of both intrinsic and extrinsic factors in the readout of cell phenotype. Thus, normally resistant B6-ROSA pyramidal neurons demonstrated an increasing sensitivity to KA, in a linear fashion, when the percentage of FVB-GFP cells was increased, either across chimeras or in different regions of the same chimera. However, the death of B6-ROSA pyramidal cells never exceeded ∼70% of the total amount of B6 neurons regardless of the amount of FVB cells in the chimeric hippocampus. In a similar manner, FVB-GFP cells show lower amounts of cell death in chimeras that are colonized by B6-ROSA cells, but again, are never fully rescued. These data indicate that both intrinsic and extrinsic factors modulate the sensitivity of hippocampal pyramidal cells to kainic acid.

A β-synuclein mutation linked to dementia produces neurodegeneration when expressed in mouse brain

The discovery of α-synuclein (αS) mutations has made a major contribution to the understanding of the pathogenesis of α-synucleinopathies such as Parkinson's disease and dementia with Lewy bodies (DLB). In contrast, less attention has been paid to β-synuclein (βS) mutations. In this paper, we show that transgenic (tg) mice expressing DLB-linked P123H βS develop progressive neurodegeneration, as characterized by axonal swelling, astrogliosis and behavioural abnormalities, with memory disorder being more prominent than motor deficits. Furthermore, cross-breeding of P123H βS tg mice with αS tg mice, but not with αS knockout mice, greatly enhanced neurodegeneration phenotypes. These results suggest that P123H βS is pathogenic and cooperates with pathogenic αS to stimulate neurodegeneration in mouse brain, indicating a causative role of P123H βS in familial DLB. Given the neuritic pathology of βS in sporadic α-synucleinopathies, it appears that alteration of βS can contribute to the pathogenesis of a broad range of α-synucleinopathies.

Lessons learnt from animal models: pathophysiology of neuropathic lysosomal storage disorders

Approximately 50 inborn errors of metabolism known as lysosomal storage disorders have been discovered to date, most of which are due to a single mutation in a gene encoding a soluble lysosomal enzyme. Consequently, inadequate enzyme activity results in the accumulation of substrates for that enzyme, invariably accompanied by a wide variety of secondary pathological changes. Many of these conditions remain untreatable, and therefore, research into pathogenic processes and potential treatment strategies is intense. A key tool for researchers in this area is the availability of clinically relevant animal models in which to study disease manifestation and evaluate therapeutic outcomes. Large numbers of both naturally occurring and genetically modified animal models of neurodegenerative lysosomal storage disorders are in existence, with spontaneous models occurring in both large domestic (e.g., cat, dog, sheep) and small (e.g., mouse) animal species. Many have undergone rigorous phenotypic characterization and are now providing us with insights into neurological disease processes. The purpose of this review is to highlight some of the major lessons learnt from these studies.

BAG1 promotes axonal outgrowth and regeneration in vivo via Raf-1 and reduction of ROCK activity

Improved survival of injured neurons and the inhibition of repulsive environmental signalling are prerequisites for functional regeneration. BAG1 (Bcl-2-associated athanogene-1) is an Hsp70/Hsc70-binding protein, which has been shown to suppress apoptosis and enhance neuronal differentiation. We investigated BAG1 as a therapeutic molecule in the lesioned visual system in vivo. Using an adeno-associated viral vector, BAG1 (AAV.BAG1) was expressed in retinal ganglion cells (RGC) and then tested in models of optic nerve axotomy and optic nerve crush. BAG1 significantly increased RGC survival as compared to adeno-associated viral vector enhanced green fluorescent protein (AAV.EGFP) treated controls and this was independently confirmed in transgenic mice over-expressing BAG1 in neurons. The numbers and lengths of regenerating axons after optic nerve crush were also significantly increased in the AAV.BAG1 group. In pRGC cultures, BAG1-over-expression resulted in a approximately 3-fold increase in neurite length and growth cone surface. Interestingly, BAG1 induced an intracellular translocation of Raf-1 and ROCK2 and ROCK activity was decreased in a Raf-1-dependent manner by BAG1-over-expression. In summary, we show that BAG1 acts in a dual role by inhibition of lesion-induced apoptosis and interaction with the inhibitory ROCK signalling cascade. BAG1 is therefore a promising molecule to be further examined as a putative therapeutic tool in neurorestorative strategies.

Axotomy-induced dopaminergic neurodegeneration is accompanied with c-Jun phosphorylation and activation transcription factor 3 expression

Accumulating evidence has shown that both phosphorylated c-Jun (pc-Jun) and activating transcription factor 3 (ATF3) were upregulated in a variety of tissue injuries and proposed to play an important role in cell death/survival. To elucidate the significance and functional role of these immediate-early genes during neuronal damage in the central nervous system, we examined temporal and spatial profiles of pc-Jun and ATF3 in dopaminergic neurons of the substantia nigra (SN) following transection of the medial forebrain bundle (MFB) in adult rats. Morphological characteristics of pc-Jun-positive dopaminergic neurons as well as microglial reaction in response to the axotomy-induced neurodegeneration were also investigated. Following MFB transection, both c-Jun phosphorylation and ATF3 were found in the nuclei of tyrosine hydroxylase-immunoreactive (TH-ir) neurons of the ipsilateral SN, but not in those of the contralateral SN. In the ipsilateral SN, the number of pc-Jun- and ATF3-positive nuclei was increased by 5-7 days post-lesion, and then progressively decreased probably due to the loss of neurons. Retrograde tracing with FluoroGold (FG) in hemi-axotomized rat brain demonstrated that none of the intact, unaxotomized (FG-ir) neurons was pc-Jun-positive, indicating phosphorylation of c-Jun occurs only in axotomized neurons. Concomitant co-localization of pc-Jun and ATF3 in the same TH-ir neuron was also demonstrated by triple immunofluorescence labeling. Many TH-ir neurons that underwent various steps of consecutive neurodegenerative changes retained pc-Jun in the condensed or fragmented nuclei. Moreover, numerous activated microglia, identified by both phagocytic (ED1) and MHC II (OX6) markers, closely apposed to these neurons throughout the entire neurodegenerative process, suggesting that they are actively phagocytosing dying neurons. Taken together, these results support the idea that pc-Jun and its putative dimeric partner ATF3 may be closely participating in axotomy-induced neurodegeneration.

Temporal mRNA profiles of inflammatory mediators in the murine 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrimidine model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). With the exception of a few rare familial forms of the disease, the precise molecular mechanisms underlying PD are unknown. Inflammation is a common finding in the PD brain, but due to the limitation of postmortem analysis its relationship to disease progression cannot be established. However, studies using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD have also identified inflammatory responses in the nigrostriatal pathway that precede neuronal degeneration in the SNpc. To assess the pathological relevance of these inflammatory responses and to identify candidate genes that might contribute to neuronal vulnerability, we used quantitative reverse-transcription polymerase chain reaction (qRT-PCR) to measure mRNA levels of 11 cytokine and chemokine encoding genes in the striatum of MPTP-sensitive (C57BL/6J) and MPTP-insensitive (Swiss Webster, SWR) mice following administration of MPTP. The mRNA levels of all 11 genes changed following MPTP treatment, indicating the presence of inflammatory responses in both strains. Furthermore, of the 11 genes examined only 3, interleukin 6 (Il-6), macrophage inflammatory protein 1 alpha/CC chemokine ligand 3 (Mip-1alpha/Ccl3) and macrophage inflammatory protein 1 beta/CC chemokine ligand 4 (Mip-1beta/Ccl4), were differentially regulated between C57BL/6J and SWR mice. In both mouse strains, the level of monocyte chemoattractant protein 1/CC chemokine ligand 2 (Mcp-1/Ccl2) mRNA was the first to increase following MPTP administration, and might represent a key initiating component of the inflammatory response. Using Mcp-1/Ccl2 knockout mice backcrossed onto a C57BL/6J background we found that MPTP-stimulated Mip-1alpha/Ccl3 and Mip-1beta/Ccl4 mRNA expression was significantly lower in the knockout mice; suggesting that Mcp-1/Ccl2 contributes to MPTP-enhanced expression of Mip-1alpha/Ccl3 and Mip-1beta/Ccl4. However, stereological analysis of SNpc neuronal loss in Mcp-1/Ccl2 knockout and wild-type mice showed no differences. These findings suggest that it is the ability of dopaminergic SNpc neurons to survive an inflammatory insult, rather than genetically determined differences in the inflammatory response itself, that underlie the molecular basis of MPTP resistance.

Swim-test as a function of motor impairment in MPTP model of Parkinson's disease: A comparative study in two mouse strains

Parkinson's disease (PD) is a common neurodegenerative disease that exhibits motor dysfunctions, such as tremor, akinesia and rigidity. In the present study, we investigated whether swim-test could be used as one of the behavioural monitoring techniques to study motor disability in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism in two mouse strains, Balb/c and C57BL/6. Mice were treated with different doses of MPTP (10, 20 and 30 mg/kg, twice, 16 h apart), and were subjected to swim-test on the third day of the first MPTP injection. MPTP-induced tremor was monitored at 30 min, and akinesia and rigidity developed were studied 3 h after the second MPTP treatment. While tremor and akinesia produced were dose-dependent and the intensity of tremor was comparable in the two strains of mice studied, the latter response in C57BL/6 was significantly lesser than that observed in Balb/c. Rigidity exhibited in Balb/c mice were dose-dependent, but not in C57BL/6. There was observed an inverse relationship between swim-score and the doses of MPTP in both the strains. MPTP caused a significant and dose-dependent reduction in striatal dopamine level in both the strains of mice, when assayed on the fourth day employing an HPLC with electrochemical detector. A significant positive correlation existed (r = 0.94 for Balb/c and r = 0.82 for C57BL/6) for the striatal dopamine-depletion and the swim-score in the MPTP-treated mice. While swim deficit and striatal dopamine loss were long lasting (till the third week) in C57BL/6, in Balb/c mice the motor deficit showed recovery by the second week. In these animals, a significant attenuation in striatal dopamine loss was observed by the third week. These results indicate that swim ability is directly proportional to striatal dopamine content, and suggest that swim-test could be used as a major technique to monitor motor dysfunction in experimental animals.

Susceptibility to excitotoxic and metabolic striatal neurodegeneration in the mouse is genotype dependent

Previously, we had reported that hippocampal susceptibility to the neurotoxic effects of excitotoxin administration is strain dependent [Schauwecker and Steward, Proc. Natl. Acad. Sci. U.S.A. 94 (1997) 4103]. However, it has been unclear whether strain-related gene products may play a similar role in providing protection against drugs that produce striatal lesions. The present series of experiments sought to elucidate whether genetic background alters neuronal viability within the striatum following metabolic or excitotoxic injury. Thus, we have examined the effect of mouse strain on susceptibility to striatal injury using well-characterized animal models of Huntington's disease by examining whether C57BL/6 mice, previously identified as resistant to excitotoxin-induced hippocampal cell death, are resistant to quinolinate, malonate, and 3-nitropropionic acid (3-NP). Intrastriatal injection of either malonate or quinolinate and systemic administration of 3-NP resulted in significantly smaller striatal lesions in C57BL/6 mice as compared to FVB/N mice, previously identified as susceptible to hippocampal excitotoxic injury. The existence of an animal strain with decreased resistance to striatal lesions suggests that there are mediating factors involved in the preferential vulnerability of the striatum to neurotoxic lesioning. The identification of these factors could provide strategies for therapeutic intervention in Huntington's disease.