Regionally-specific microglial activation in young mice over-expressing human wildtype alpha-synuclein.

Parkinson's disease (PD) is characterized by widespread alpha-synuclein pathology and neuronal loss, primarily of the nigrostriatal dopaminergic neurons. Inflammation has been implicated in PD, and alpha-synuclein can initiate microglial activation; however, the kinetics and distribution of inflammatory responses to alpha-synuclein overexpression in vivo are not well understood. We have examined the regional and temporal pattern of microglial activation and pro-inflammatory cytokine production in mice over-expressing wild-type human alpha-synuclein driven by the Thy1-promoter (Thy1-aSyn mice). An increased number of activated microglia, and increased levels of TNF-α mRNA and protein were first detected in the striatum (1 month of age) and later in the substantia nigra (5-6 months), but not the cerebral cortex or cerebellum; in contrast, IL-1ß and TGF-ß remained unchanged in the striatum and substantia nigra at all ages examined. Microglial activation persisted up to 14 months of age in these regions and only minimal increases were observed in other regions at this later age. Increased concentrations of serum TNF-α were observed at 5-6 months, but not at 1 month of age. The expression of toll-like receptors (TLRs) 1, TLR 4 and TLR 8, which are possible mediators of microglial activation, was increased at 5-6 months in the substantia nigra but not in the cerebral cortex, and TLR 2 was increased in the substantia nigra at 14 months of age. With the exception of a slight increase in the striatum of 14 month old Thy1-aSyn mice, MHCII staining was not detected in the regions and ages examined. Similarly, peripheral CD4 and CD8-postive T cells were increased in the blood but only at 22 months of age, suggesting later involvement of the adaptive immune response. These data indicate that, despite the presence of high levels of alpha-synuclein in other brain regions, alpha-synuclein overexpression caused a selective early inflammatory response in regions containing the axon terminals and cell bodies of the nigrostriatal pathway. Our results suggest that specific factors, possibly involving a regionally and temporally selective increase in TLRs, mediate alpha-synuclein-induced inflammatory responses in the SN, and may play a role in the selective vulnerability of nigrostriatal dopaminergic neurons in PD.

A progressive mouse model of Parkinson's disease: the Thy1-aSyn ("Line 61") mice.

Identification of mutations that cause rare familial forms of Parkinson's disease (PD) and subsequent studies of genetic risk factors for sporadic PD have led to an improved understanding of the pathological mechanisms that may cause nonfamilial PD. In particular, genetic and pathological studies strongly suggest that alpha-synuclein, albeit very rarely mutated in PD patients, plays a critical role in the vast majority of individuals with the sporadic form of the disease. We have extensively characterized a mouse model over-expressing full-length, human, wild-type alpha-synuclein under the Thy-1 promoter. We have also shown that this model reproduces many features of sporadic PD, including progressive changes in dopamine release and striatal content, alpha-synuclein pathology, deficits in motor and nonmotor functions that are affected in pre-manifest and manifest phases of PD, inflammation, and biochemical and molecular changes similar to those observed in PD. Preclinical studies have already demonstrated improvement with promising new drugs in this model, which provides an opportunity to test novel neuroprotective strategies during different phases of the disorder using endpoint measures with high power to detect drug effects.

Interleukin-1alpha and HMGB1 mediate hippocampal dysfunction in SIGIRR-deficient mice.

Single-Ig-interleukin-1 related receptor (SIGIRR) is a member of the interleukin (IL)-1/Toll-like receptor (TLR) family. It negatively regulates inflammation, rendering SIGIRR(-/-) mice more susceptible to inflammatory challenge. This susceptibility extends to the brain, where increased responsiveness to lipopolysaccharide has been observed in SIGIRR-deficient mice. While this is likely due to enhanced TLR4-mediated signaling, the functional consequences of these changes have not yet been described. In the current study, we have investigated the impact of SIGIRR deficiency on hippocampal function, and show that novel object recognition, spatial reference memory, and long-term potentiation (LTP) were impaired in SIGIRR(-/-) mice. These changes were accompanied by increased expression of IL-1RI and TLR4, and upregulation of their downstream signaling events, namely IRAK1 (IL-1R-associated kinase 1), c-Jun N-terminal protein kinase (JNK), and nuclear factor κB (NF-κB). The deficit in LTP was attenuated by the endogenous IL-1 receptor antagonist (IL-1ra) and an anti-TLR4 antibody, and also by inhibition of JNK and NF-κB. We propose that IL-1RI is activated by IL-1α and TLR4 is activated by the endogenous agonist, high mobility group box 1 (HMGB1), as we identified enhanced expression of both cytokines in the hippocampus of SIGIRR(-/-) mice. Additionally, application of HMGB1 increased the activation of JNK and NF-κB and was found to be detrimental to LTP in a TLR4-dependent manner. These findings highlight the functional role of SIGIRR in regulating inflammatory-mediated synaptic and cognitive decline, and describe evidence of the key role of HMGB1 in this process.

SIGIRR modulates the inflammatory response in the brain.

One of the more recently described members of the interleukin-1 (IL-1) receptor family, single-Ig-interleukin-1 related receptor (SIGIRR), has been identified as a negative regulator of inflammation in several tissues. It modulates the responses triggered by stimulation of Toll-like receptor (TLR) 4 and IL-1 in several peripheral cell types, possibly in an NFkappaB-dependent manner. Consistently, responses to lipopolysaccharide (LPS) are exaggerated in SIGIRR-deficient mice and the symptoms of experimental inflammatory conditions are more profound in these animals. Here, we set out to establish whether the absence of SIGIRR was associated with inflammatory changes in the brain and report that, LPS-induced a greater effect on CD40 and ICAM mRNA in mixed glia prepared from SIGIRR(-/-), compared with wildtype mice. This was associated with parallel changes in TNFalpha and IL-6 at mRNA and protein levels, an effect which was observed in purified microglia but not astrocytes. Similarly, LPS exerted a more profound effect on microglial activation and cytokine production in hippocampal tissue prepared from SIGIRR(-/-), compared with wildtype mice. The effect of LPS on exploratory behaviour was also accentuated in SIGIRR(-/-) mice. The evidence suggests that these changes are a likely consequence of increased hippocampal expression of CD14 and TLR4, and NFkappaB activation in SIGIRR(-/-) mice.