Regulation of GAP-43 protein and mRNA in nigrostriatal dopaminergic neurons after the partial destruction of dopaminergic terminals with intrastriatal 6-hydroxydopamine

High Fat Diet Suppresses Peroxisome Proliferator-Activated Receptors and Reduces Dopaminergic Neurons in the Substantia Nigra

Although several epidemiologic and animal studies have revealed correlations between obesity and neurodegenerative disorders, such as Parkinson disease (PD), the underlying pathological mechanisms of obesity-induced PD remain unclear. Our study aimed to assess the effect of diet-induced obesity on the brain dopaminergic pathway. For five months, starting from weaning, we gave C57BL/6 mice a high-fat diet (HFD) to generate an obese mouse model and investigate whether the diet reprogrammed the midbrain dopaminergic system. Tyrosine hydroxylase staining showed that the HFD resulted in fewer dopaminergic neurons in the substantia nigra (SN), but not the striatum. It also induced neuroinflammation, with increased astrogliosis in the SN and striatum. Dendritic spine density in the SN of HFD-exposed mice decreased, which suggested that prolonged HFD altered dopaminergic neuroplasticity. All three peroxisome proliferator-activated receptor (PPAR) subtype (PPAR-α, PPAR-β/δ, PPAR-γ) levels were significantly reduced in the SN and the ventral tegmental area of HFD mice when compared to those in controls. This study showed that a prolonged HFD induced neuroinflammation, suppressed PPAR levels, caused degeneration of midbrain dopaminergic neurons, and resulted in symptoms reminiscent of human PD. To our knowledge, this is the first study documenting the effects of an HFD on PPARs in dopaminergic neurons.

Evidence for cross-hemispheric preconditioning in experimental Parkinson's disease

Dopamine loss and motor deficits in Parkinson's disease typically commence unilaterally and remain asymmetric for many years, raising the possibility that endogenous defenses slow the cross-hemispheric transmission of pathology. It is well-established that the biological response to subtoxic stress prepares cells to survive subsequent toxic challenges, a phenomenon known as preconditioning, tolerance, or stress adaptation. Here we demonstrate that unilateral striatal infusions of the oxidative toxicant 6-hydroxydopamine (6-OHDA) precondition the contralateral nigrostriatal pathway against the toxicity of a second 6-OHDA infusion in the opposite hemisphere. 6-OHDA-induced loss of dopaminergic terminals in the contralateral striatum was ablated by cross-hemispheric preconditioning, as shown by two independent markers of the dopaminergic phenotype, each measured by two blinded observers. Similarly, loss of dopaminergic somata in the contralateral substantia nigra was also abolished, according to two blinded measurements. Motor asymmetries in floor landings, forelimb contacts with a wall, and spontaneous turning behavior were consistent with these histological observations. Unilateral 6-OHDA infusions increased phosphorylation of the kinase ERK2 and expression of the antioxidant enzyme CuZn superoxide dismutase in both striata, consistent with our previous mechanistic work showing that these two proteins mediate preconditioning in dopaminergic cells. These findings support the existence of cross-hemispheric preconditioning in Parkinson's disease and suggest that dopaminergic neurons mount impressive natural defenses, despite their reputation as being vulnerable to oxidative injury. If these results generalize to humans, Parkinson's pathology may progress slowly and asymmetrically because exposure to a disease-precipitating insult induces bilateral upregulation of endogenous defenses and elicits cross-hemispheric preconditioning.

Altered Expression of Growth Associated Protein-43 and Rho Kinase in Human Patients with Parkinson's Disease

Causative treatment strategies for Parkinson's disease (PD) will have to address multiple underlying pathomechanisms to attenuate neurodegeneration. Additionally, the intrinsic regenerative capacity of the central nervous system is also an important factor contributing to restoration. Extracellular cues can limit sprouting and regrowth of adult neurons, but even aged neurons have a low intrinsic regeneration capacity. Whether this capacity has been lost or if growth inhibitory cues are increased during PD progression has not been resolved yet. In this study, we assessed the regenerative potential in the nigrostriatal system in post-mortem brain sections of PD patients compared to age-matched and young controls. Investigation of the expression pattern of the regeneration-associated protein GAP-43 suggested a lower regenerative capacity in nigral dopaminergic neurons of PD patients. Furthermore, the increase in protein expression of the growth-inhibitory protein ROCK2 in astrocytes and a similar trend in microglia, suggests an important role for ROCK2 in glial PD pathology, which is initiated already in normal aging. Considering the role of astro- and microglia in PD pathogenesis as well as beneficial effects of ROCK inhibition on neuronal survival and regeneration in neurodegenerative disease models, our data strengthens the importance of the ROCK pathway as a therapeutic target in PD.

Alpha-synuclein mutations impair axonal regeneration in models of Parkinson's disease

The dopaminergic (DAergic) nigrostriatal tract has an intrinsic regenerative capacity which can be impaired in Parkinson's disease (PD). Alpha-synuclein (aSyn) is a major pathogenic component in PD but its impact on DAergic axonal regeneration is largely unknown. In this study, we expressed pathogenic variants of human aSyn by means of recombinant adeno-associated viral vectors in experimental paradigms of DAergic regeneration. In a scratch lesion model in vitro, both aSyn(A30P) and aSyn(A53T) significantly reduced DAergic neurite regeneration and induced loss of TH-immunopositive cells while aSyn(WT) showed only minor cellular neurotoxic effects. The striatal density of TH-immunopositive axons in the striatal 6-OHDA lesion mouse model was attenuated only by aSyn(A30P). However, striatal expression levels of the regeneration marker GAP-43 in TH-immunopositive fibers were reduced by both aSyn(A30P) and aSyn(A53T), but not by aSyn(WT), which was associated with an activation of the ROCK signaling pathway. Nigral DAergic cell loss was only mildly enhanced by additional overexpression of aSyn variants. Our findings indicate that mutations of aSyn have a strong impact on the regenerative capacity of DAergic neurons, which may contribute to their pathogenic effects.

Increase of GAP-43 in the rat cerebellum following unilateral striatal 6-OHDA lesion

In order to further characterize synaptic alterations following a severe lesion of the nigrostriatal system, the expression of synaptic marker proteins, synaptophysin and growth-associated protein-43 (GAP-43), was examined in various brain regions of 6-hydroxydopamine (6-OHDA)-treated rats, an animal model of Parkinson's disease. Unilateral nigrostriatal lesioning induced an increase in synaptophysin protein levels by 68% and 106% in the sensorimotor cortex and striatum, respectively, while changes in the level of GAP-43 were not observed. In contrast, 6-OHDA induced a 73% increase in the level of GAP-43 protein in the cerebellum. This increase was also confirmed with immunohistochemistry. The level of synaptophysin in the cerebellum remained unchanged in response to the lesion. These results suggest that a neurotoxic lesion of the nigrostriatal pathway differentially affects the expression of the two synaptic proteins and that plasticity-related changes in this model are not solely restricted to the nigrostriatal system. In addition, these results provide further evidence of the involvement of the cerebellum in the late response to a 6-OHDA lesion.

Rapid increase of Nurr1 expression in the substantia nigra after 6-hydroxydopamine lesion in the striatum of the rat

Nurr1 is a transcription factor essential for the genesis of ventral dopaminergic neurons. In this study, we investigated the expression of Nurr1 protein and mRNA in the adult rat brain by using immunohistochemistry and in situ hybridization, respectively. Another aim of our study was to investigate Nurr1 expression in substantia nigra after dopamine depletion induced by the injection of 6-hydroxydopamine in the striatum. We observed that Nurr1 mRNA and protein are expressed in several brain regions, including cortex, hippocampus, substantia nigra, and ventral tegmental area, in agreement with previous reports using in situ hybridization. Additionally, we found that Nurr1 is expressed in brain regions that have not been previously reported, such as striatum, septum, and superior colliculus. Highest levels of expression were found in cortex, medial septum, dentate gyrus, some hypothalamic nuclei, and substantia nigra. Interestingly, we observed that, in the superior colliculus, Nurr1 protein is localized in the cytoplasm of cells, whereas, in other regions, it was localized mainly in the nuclei, suggesting that Nurr1 subcellular localization is regulated and may have functional implications. Dopamine depletion induced by an injection of 6-hydroxydopamine into the striatum produced an increase in the number of cells expressing Nurr1 mRNA and protein in both substantia nigra compacta and substantia nigra reticulata, ipsilateral and contralateral to the lesioned side, measured 24 hr after the 6-hydroxydopamine injection. These results suggest that Nurr1 may be involved in many neuronal functions in the adult central nervous system and, in particular, might be related to the compensation processes that take place in dopaminergic cells in order to normalize extracellular dopamine levels in the striatum.

Gene expression profiling in the midbrain of striatal 6-hydroxydopamine-injected mice

In order to clarify mechanisms underlying dopaminergic neuronal death in Parkinson's disease (PD), a gene expression profiling study was performed in a rodent model of PD. In this model, mice are intrastriatally injected with 6-hydroxydopamine (6-OHDA) and dopaminergic neurons in the substantia nigra (SN) gradually die by retrograde degeneration. The SN were removed 2 h, 24 h, or 14 days after 6-OHDA administration. Levels of mRNAs related to cell death or survival were quantified using adaptor-tagged competitive PCR (ATAC-PCR). The cyclin D1 gene showed an immediate increase in mRNA expression. After 24 h, when dopaminergic neurons were under intense degeneration, levels of caspase 8 mRNA and p53 apoptosis effecter related to pmp 22 (PERP) mRNA increased and, conversely, FAS mRNA decreased. After 14 days, when the degeneration was attenuated, levels of PERP mRNA and serum- and glucocorticoid-regulated kinase (SGK) mRNA still increased. SGK has a similarity to AKT, which is an important molecule involved in nerve growth factor signal transduction. AKT mRNA levels are low in dopaminergic neurons. These results suggest that an increase in cyclin D1 mRNA triggers dopaminergic neurons to enter an abnormal cell cycle, which leads to neuronal degeneration and cell death, possibly induced by PERP and caspase 8. In addition to cell death-related genes, several survival-related genes are activated. SGK might function as a key enzyme for the survival of dopaminergic neurons.