Inflammation and α-synuclein's prion-like behavior in Parkinson's disease--is there a link?

Parkinson's disease patients exhibit progressive spreading of aggregated α-synuclein in the nervous system. This slow process follows a specific pattern in an inflamed tissue environment. Recent research suggests that prion-like mechanisms contribute to the propagation of α-synuclein pathology. Little is known about factors that might affect the prion-like behavior of misfolded α-synuclein. In this review, we suggest that neuroinflammation plays an important role. We discuss causes of inflammation in the olfactory bulb and gastrointestinal tract and how this may promote the initial misfolding and aggregation of α-synuclein, which might set in motion events that lead to Parkinson's disease neuropathology. We propose that neuroinflammation promotes the prion-like behavior of α-synuclein and that novel anti-inflammatory therapies targeting this mechanism could slow disease progression.

Alpha-synuclein cell-to-cell transfer and seeding in grafted dopaminergic neurons in vivo.

Several people with Parkinson's disease have been treated with intrastriatal grafts of fetal dopaminergic neurons. Following autopsy, 10-22 years after surgery, some of the grafted neurons contained Lewy bodies similar to those observed in the host brain. Numerous studies have attempted to explain these findings in cell and animal models. In cell culture, α-synuclein has been found to transfer from one cell to another, via mechanisms that include exosomal transport and endocytosis, and in certain cases seed aggregation in the recipient cell. In animal models, transfer of α-synuclein from host brain cells to grafted neurons has been shown, but the reported frequency of the event has been relatively low and little is known about the underlying mechanisms as well as the fate of the transferred α-synuclein. We now demonstrate frequent transfer of α-synuclein from a rat brain engineered to overexpress human α-synuclein to grafted dopaminergic neurons. Further, we show that this model can be used to explore mechanisms underlying cell-to-cell transfer of α-synuclein. Thus, we present evidence both for the involvement of endocytosis in α-synuclein uptake in vivo, and for seeding of aggregation of endogenous α-synuclein in the recipient neuron by the transferred α-synuclein. Finally, we show that, at least in a subset of the studied cells, the transmitted α-synuclein is sensitive to proteinase K. Our new model system could be used to test compounds that inhibit cell-to-cell transfer of α-synuclein and therefore might retard progression of Parkinson neuropathology.

Structural and functional characterization of monomeric EphrinA1 binding site to EphA2 receptor.

The EphA2 receptor is overexpressed in glioblastoma multiforme and has been to shown to contribute to cell transformation, tumor initiation, progression, and maintenance. EphrinA1 (eA1) is a preferred ligand for the receptor. Treatment with monomeric eA1, the form of eA1 found in the extracellular environment, causes receptor phosphorylation, internalization, and down-regulation with subsequent anti-tumor effects. Here, we investigated the structure-function relationship of a monomeric eA1 focusing on its G-H loop ((108)FQRFTPFTLGKEFKE(123)G), a highly conserved region among eAs that mediates binding to their receptors. Alanine substitution mutants of the G-H loop amino acids were transfected into U-251 MG glioblastoma multiforme cells, and functional activity of each mutant in conditioned media was assessed by EphA2 down-regulation, ERK and AKT activation and cellular response assays. Alanine substitutions at positions Pro-113 Thr-115, Gly-117, Glu-122, and also Gln-109 enhanced the EphA2 receptor down-regulation and decreased p-ERK and p-AKT. Substitution mutants of eA1 at positions Phe-108, Arg-110, Phe-111, Thr-112, Phe-114, Leu-116, Lys-118, Glu-119, and Phe-120 had a deleterious effect on EphA2 down-regulation when compared with eA1-WT. Mutants at positions Phe-108, Lys-18, Lys-121, Gly-123 retained similar properties to eA1-WT. Recombinant eA1-R110A, -T115A, -G117A, and -F120A have been found to exhibit the same characteristics as the ligands contained in the conditioned media mainly due to the differences in their binding to the receptor. Thus, we have identified variants of eA1 that possess either superagonistic or antagonistic properties. These new findings will be important in the understanding of the receptor/ligand interactions and in further design of anti-cancer therapies targeting the eA/EphA system.

Decline in age-dependent, MK801-induced injury coincides with developmental switch in parvalbumin expression: somatosensory and motor cortex.

MK801-induced activation of caspase-3 is developmentally regulated, peaking at postnatal day (P) 7 and decreasing with increasing postnatal age thereafter. Further, at P7, cells displaying activation of caspase-3 lack expression of calcium binding proteins (CaBPs). To further explore this relationship, we investigated postnatal expression of calbindin (CB), calretinin (CR) and parvalbumin (PV) in two brain regions susceptible to MK801-induced injury, the somatosensory cortex (S1) and layer II/III of motor cortex (M1/M2). Expression of CB and especially PV was low to absent prior to P7 but substantially increased from P7 through to P21 and adulthood. In contrast, CR expression was more variable at early developmental ages, stabilized to lower levels after P7 and showed a marked decline by P21. The results suggest that not only does calcium buffering capacity increase developmentally but also acquisition of enhanced buffering may be one mechanism by which neurons survive agent-induced alterations in calcium homeostasis.

Intraneuronal vesicular organelle transport changes with cell population density in vitro.

Primary neuron cultures are widely used in research due to the ease and usefulness of observing individual cells. Therefore, it is vital to understand how variations in culture conditions may affect neuron physiology. One potential variation for cultured neurons is a change in intracellular transport. As transport is necessary for the normal delivery of organelles, proteins, nucleic acids, and lipids, it is a logical indicator of a cell's physiology. We test the hypothesis that organelle transport may change with varying in vitro population densities, thus indicating a change in cellular physiology. Using a novel background subtraction imaging method we show that, at 5 days in vitro (DIV), transport of vesicular organelles in embryonic rat spinal cord neurons is positively correlated with cell density. When density increased 6.5-fold, the number of transported organelles increased 2.2+/-0.3-fold. Intriguingly, this effect was not observable at 3-4 DIV. These results show a significant change in cellular physiology with a relatively small change in plating procedure; this indicates that cells appearing to be morphologically similar, and at the same DIV, may still suffer from a great degree of variability.

Decline in age-dependent, MK801-induced injury coincides with developmental switch in parvalbumin expression: cingulate and retrosplenial cortex.

Age-dependent, MK801-induced, activated caspase-3 expression in the postnatal brain is generally not observed in neurons expressing calcium-binding proteins (CaBPs), suggesting that apoptosis and calcium buffering are inversely related. In regions such as the cingulate and retrosplenial cortex, injury peaks at postnatal Day 7 (P7) and rapidly diminishes thereafter, whereas expression of calbindin (CB) and calretinin (CR) was relatively low from P0 to P7 and steadily increased from P7 to P14. At ages thereafter, CB and CR expression either remained stable then declined or rapidly declined. Parvalbumin (PV) was generally low-absent prior to P7 but expression dramatically increased from P10 onwards, peaking at P21. These studies suggest calcium entry (through N-methyl-D-aspartate receptor (NMDARs)) and buffering (by CaBPs) are integral to normal CNS maturation. Because schizophrenia is associated with glutamate hypo-function, developmental injury, and aberrant CaBP expression, our data indicate that this postnatal brain injury model may offer important insights into the nature of this disorder.

Neonatal exposure to MK801 induces structural reorganization of the central nervous system.

Schizophrenia, a progressive disorder displaying widespread pathological changes, is associated with the loss of glutamatergic function and selective loss of cytoskeletal proteins, such as MAP2, in regions severely affected by this disease. As schizophrenia is associated with perinatal brain trauma, we monitored changes in several functionally different proteins following injury-promoting MK801 blockade of N-methyl-D-aspartate receptors in neonatal rats. Within the somatosensory cortex, MK801 triggered robust, caspase-3-dependent apoptotic injury, reduced expression of cytoskeletal proteins MAP2 and tau, and increased synapse associated protein SNAP25. Thus, both neuronal injury and loss of structural elements important for successful cell-cell contact may reorganize brain circuitry, which at later ages could promote similar behavioral changes observed in schizophrenia.