Peripheral-Central Neuroimmune Crosstalk in Parkinson's Disease: What Do Patients and Animal Models Tell Us?

The brain is no longer considered an immune privileged organ and neuroinflammation has long been associated with Parkinson's disease. Accumulating evidence demonstrates that innate and adaptive responses take place in the CNS. The extent to which peripheral immune alterations impacts on the CNS, or vice and versa, is, however, still a matter of debate. Gaining a better knowledge of the molecular and cellular immune dysfunctions present in these two compartments and clarifying their mutual interactions is a fundamental step in understanding and preventing Parkinson's disease (PD) pathogenesis. This review provides an overview of the current knowledge on inflammatory processes evidenced both in PD patients and in toxin-induced animal models of the disease. It discusses differences and similarities between human and animal studies in the context of neuroinflammation and immune responses and how they have guided therapeutic strategies to slow down disease progression. Future longitudinal studies are necessary and can help gain a better understanding on peripheral-central nervous system crosstalk to improve therapeutic strategies for PD

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A2A receptors (A2AR) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D2 and adenosine A2A receptors in the basal ganglia. At present it is believed that A2AR antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A2AR antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D2 receptors (D2Rs) expressed in the striatum are known to form heteromers with A2A adenosine receptors. Thus, the development of heteromer-specific A2A receptor antagonists represents a promising strategy for the identification of more selective and safer drugs

Activation of the CREB/c-Fos pathway during long-term synaptic plasticity in the cerebellum granular layer

The induction of long-term potentiation and depression (LTP and LTD) is thought to trigger gene expression and protein synthesis, leading to consolidation of synaptic and neuronal changes. However, while LTP and LTD have been proposed to play important roles for sensori-motor learning in the cerebellum granular layer, their association with these mechanisms remained unclear. Here, we have investigated phosphorylation of the cAMP-responsive element binding protein (CREB) and activation of the immediate early gene c-Fos pathway following the induction of synaptic plasticity by thetaburst stimulation (TBS) in acute cerebellar slices. LTP and LTD were localized using voltage-sensitive dye imaging (VSDi). At two time points following TBS (15 min and 120 min), corresponding to the early and late phases of plasticity, slices were fixed and processed to evaluate CREB phosphorylation (P-CREB) and c-FOS protein levels, as well as Creb and c-Fos mRNA expression. High levels of P-CREB and Creb/c-Fos were detected before those of c-FOS, as expected if CREB phosphorylation triggered gene expression followed by protein synthesis. No differences between control slices and slices stimulated with TBS were observed in the presence of an N-methyl-Daspartate receptor (NMDAR) antagonist. Interestingly, activation of the CREB/c-Fos system showed a relevant degree of colocalization with long-term synaptic plasticity. These results show that NMDAR-dependent plasticity at the cerebellum input stage bears about transcriptional and post-transcriptional processes potentially contributing to cerebellar learning and memory consolidation

Immediate early genes regulation in rat cerebellar cortex during long-term synaptic plasticity induction

The cerebellum is one of the brain areas involved in learning and memory formation. Long-term synaptic plasticity is thought to play a pivotal role in supporting these functions. Moreover Immediate Early Genes (IEGs) expression and de novo protein synthesis and/or modification have been strictly associated with maintenance of Long-Term Potentiation (LTP) as well as memory consolidation and storage. Two highly conserved signalling cascades, PKA and MAPK, seem to be involved in early- to late-LTP conversion; both pathway can activate CREB transcription factor through phosphorylation and P-CREB has been suggested to initiate the protein synthesis leading to late-LTP induction. The transcription factor c-fos is known to be rapidly and transiently induced in the Nervous System by a variety of stimuli and is thought to be directly involved in processes of neuronal plasticity including LTP. We used rat parasagittal cerebellar slices as a model system in which specific patterns of stimulation delivered to the mossy fibers can induce both Long-Term Potentiation and Long-Term Depression (LTD), depending on local inhibition and other regulating factors. Using Voltage Sensitive Dye (VSD) imaging we obtained high-resolution maps of the spatial distribution of LTP/LTD induced from a Teta Burst Stimulus (TBS) application. Control and stimulated slices were fixed at different times from the TBS application and processed for in situ hybridization or immunohystochemistry in order to detect IEGs mRNA expression patterns and protein expression/modifications. The expression pattern of c-fos and CREB mRNAs and their protein distribution and/or phosphorylation were then correlated with LTP/LTD maps generated by VSD imaging. Preliminary data indicate a significant increase of P-CREB in the granular layer suggesting that CREB phosphorylation is induced as early as 15 minutes post TBS application. In situ hybridization experiments indicate a good correlation between c-fos and CREB mRNAs up-regulation and LTP distribution at 120 minutes post TBS. At the protein level, the comparison of immunofluorescence signals and VSD immaging data indicate a clear correlation between c-Fos and P-CREB distribution and synaptic plasticity patterns. We are planning further experiments to confirm these data and to test our experimental system in the presence of drugs that could interfere with the transcription, translation or post-translational protein regulation

Immediate early genes expression in the cerebellar cortex correlates with LTP and LTD induction

The consolidation of changes following activity-dependent neural plasticity are believed to involve specific patterns of gene expression. In the hippocampus, immediate early genes are thought to contribute to long-term synaptic plasticity (LTP and LTD); this phenomenon may occur also in the cerebellum, in which the transcription factors c-Fos and P-CREB have been identified. The cerebellum granular layer (GL) can manifest both LTP and LTD following a Theta Burst Stimulus (TBS) delivered to the mossy fibers. We have employed VSD imaging in rat cerebellar slices (P18-24) in order to map the spatial distribution of LTP and LTD in the cerebellum GL. Fluorescence changes were correlated to LTP or LTD in two different post-TBS time ranges (15 and 120 min). Slices were then fixed and processed for immunohistochemistry in order to identify levels of c-Fos and P-CREB expression. The induction of long-term plasticity increased the average level of P-CREB both at 15 min (+39±4.9, p<0.01%) and 120 min (+24±7.2, p<0.05%) after TBS. The level of c-Fos was unaltered at 15 min, while it significantly increased at 120 min (+37±8.9, p<0.05%). By spatially correlating longterm synaptic plasticity with the corresponding variation of P-CREB and c-Fos, we observed that regions showing LTP well correlated (p<0.05) with positive variations of P-CREB and c-Fos. Conversely, areas showing LTD correlated exclusively (p<0.05) with negative variations of P-CREB. Slices were also evaluated by in situ hybridization and a similar analysis was performed. The levels of fos and CREB mRNA expression and their spatial correlation with the sign of long-term synaptic plasticity corresponded with the immunohistochemical results. As a further test, VSD recordings showed that the induction of granular layer LTP and LTD could be prevented by applying 50 mM D-APV, a selective NMDA receptor blocker. Moreover, in situ hybridization and immunohistochemistry analysis evidenced that in these conditions both mRNA and protein expression levels of c-fos and CREB were unchanged, confirming the involvement of these two transcription factors in cerebellar granular layer plasticity