[Is Parkinson's disease a prion disease?]

The accumulation of a specific protein in aggregated form is a common phenomenon in human neurodegenerative diseases. In Parkinson's disease, this protein is α-synuclein which is a neuronal protein of 143 amino acids. With a monomeric conformation in solution, it also has a natural capacity to aggregate into amyloid structures (dimers, oligomers, fibrils and Lewy bodies or neurites). It therefore fulfils the characteristics of a prion protein (different conformations, seeding and spreading). In vitro and in vivo experimental evidence in transgenic and wild animals indicates a prion-like propagation of Parkinson's disease. The sequential and predictive distribution of α-synuclein demonstrated by Braak et al. and its correlation with non-motor signs are consistent with the prion-like progression. Although the triggering factor causing the misfolding and aggregation of the target protein is unknown, Parkinson's disease is a highly relevant model for the study of these mechanisms and also to test specific treatments targeting the assemblies of α-synuclein and propagation from pre-motor phase of the disease. Despite this prion-like progression, there is currently no argument indicating a risk of human transmission of Parkinson's disease.

Quantification of surviving cerebellar granule neurones and abnormal prion protein (PrPSc) deposition in sporadic Creutzfeldt-Jakob disease supports a pathogenic role for small PrPSc deposits common to the various molecular subtypes

AIMS

Neuronal death is a major neuropathological hallmark in prion diseases. The association between the accumulation of the disease-related prion protein (PrP(Sc) ) and neuronal loss varies within the wide spectrum of prion diseases and their experimental models. In this study, we investigated the relationships between neuronal loss and PrP(Sc) deposition in the cerebellum from cases of the six subtypes of sporadic Creutzfeldt-Jakob disease (sCJD; n=100) that can be determined according to the M129V polymorphism of the human prion protein gene (PRNP) and PrP(Sc) molecular types.

METHODS

The numerical density of neurones was estimated with a computer-assisted image analysis system and the accumulation of PrP(Sc) deposits was scored.

RESULTS

The scores of PrP(Sc) immunoreactive deposits of the punctate type (synaptic type) were correlated with neurone counts - the higher the score the higher the neuronal loss - in all sCJD subtypes. Large 5- to 50-µm-wide deposits (focal type) were found in sCJD-MV2 and sCJD-VV2 subtypes, and occasionally in a few cases of the other studied groups. By contrast, the highest scores for 5- to 50-µm-wide deposits observed in sCJD-MV2 subtype were not associated with higher neuronal loss. In addition, these scores were inversely correlated with neuronal counts in the sCJD-VV2 subtype.

CONCLUSIONS

These results support a putative pathogenic role for small PrP(Sc) deposits common to the various sCJD subtypes. Furthermore, the observation of a lower loss of neurones associated with PrP(Sc) type-2 large deposits is consistent with a possible 'protective' role of aggregated deposits in both sCJD-MV2 and sCJD-VV2 subtypes.

Tauopathy in human and experimental variant Creutzfeldt-Jakob disease

Cerebral accumulation of hyperphosphorylated tau (phospho-tau) occurs in several neurodegenerative conditions including Alzheimer disease. In prion diseases, phospho-tau deposition has been described in a rare genetic form, Gerstmann-Sträussler-Scheinker disease, but is not considered part of the neuropathological picture of Creutzfeldt-Jakob disease. Aim of this study was to investigate whether changes related to phospho-tau accumulation are present in the brain of patients with variant Creutzfeldt-Jakob disease (vCJD) that shares with Gerstmann-Sträussler-Scheinker disease abundant prion protein (PrP) deposition in amyloid form. The analysis was extended to experimental mouse models of vCJD. We detected a large number of phospho-tau-immunoreactive neuritic profiles, often clustered around PrP amyloid deposits, not only in the cerebral cortex, but also in the cerebellum of all vCJD patients examined, in the absence of Abeta. Although less constantly, phospho-tau was localized in some perikaria and dendrites. The biochemical counterpart was the presence of phospho-tau in the detergent-insoluble fraction of cerebral cortex. Phospho-tau-immunoreactive neuronal profiles were also found in association with PrP deposits in mouse models of vCJD. These findings suggest that the abnormal forms of PrP associated with vCJD trigger a tauopathy, and provide a paradigm for the early stages of tau pathology associated with cerebral amyloidoses, including Alzheimer disease.

A novel mutation (G114V) in the prion protein gene in a family with inherited prion disease

Inherited prion diseases are characterized by mutations in the PRNP gene encoding the prion protein (PrP). We report a novel missense mutation in the PRNP gene (resulting in a G114V mutation in PrP) in members of a Uruguayan family with clinical and histopathologic features of prion disease. Affected individuals were characterized by an early age at onset, initial neuropsychiatric symptoms, late dementia with prominent pyramidal and extrapyramidal symptoms, and long disease duration.

Compassionate use of quinacrine in Creutzfeldt–Jakob disease fails to show significant effects

Quinacrine has been reported as an antiprion agent and proposed as an immediately applicable treatment for Creutzfeldt–Jakob disease (CJD). The authors report the results of an open compassionate procedure to which 32 CJD patients had access. In some genotypic subgroups, a slight but nonsignificant increase in survival was observed, likely due to biased inclusion of long-term surviving patients. There was no pathologic evidence of a beneficial effect of quinacrine treatment.

Alpha-synuclein-immunoreactive deposits in human and animal prion diseases

Prion related disorders are associated with the accumulation of a misfolded isoform (PrPsc) of the host-encoded prion protein, PrP. There is strong evidence for the involvement of unidentified co-factors in the PrP to PrPsc conversion process. In this study, we show alpha-synuclein-immunoreactive deposits in the central nervous system of various prion diseases (sporadic, iatrogenic and new variant Creutzfeldt-Jakob diseases, and experimental scrapie of hamsters). alpha-Synuclein accumulated close to PrPsc deposits but we did not observe strict colocalization of prion protein and alpha-synuclein immunoreactivities particularly in PrPsc plaques. alpha-Synuclein is thought to be a key player in some neurodegenerative disorders, is able to interact with amyloid structures and has known chaperone-like activities. Our results, in various prion diseases, suggest a role for alpha-synuclein in regulating PrPsc formation.

Neurotoxicity of the putative transmembrane domain of the prion protein

It has been shown recently that the generation of an abnormal transmembrane form of the prion protein ((Ctm)PrP) is involved in the neurodegeneration process during inherited and infectious prion diseases but a causative relationship has never been established. We wanted to know if and how the proposed transmembrane domain of PrP could induce neuronal dysfunction. Thus, we investigated the neurotoxic properties of two peptides whose sequences are encompassed within this domain. We show that PrP peptides 118-135 and 105-132 as well as an amidated more soluble peptide 105-132 induce the death of pure cortical neurons originating from normal and PrP knockout mice. This can be correlated with the high propensity of these peptides to insert stably into and to destabilize cell membranes. Through this study, we have identified a novel mechanism of neurotoxicity for PrP, which directly involves membrane perturbation; this mechanism is independent of fibril formation and probably corresponds to the effect of the transmembrane insertion of (Ctm)PrP.

Dementia with Lewy bodies in a neuropathologic series of suspected Creutzfeldt-Jakob disease

Discriminating Creutzfeldt-Jakob disease (CJD) from dementia with Lewy bodies (DLB) may be clinically difficult to achieve. The authors describe 10 patients with DLB initially referred to the French Network of Human Spongiform Encephalopathies as having suspected CJD. In a series of 465 autopsied cases, DLB ranked second among degenerative alternative diagnoses to CJD. The authors analyzed the factors that contributed to misleading the diagnosis, and suggest that the detection of 14-3-3 protein in CSF may be useful to distinguish CJD from DLB.

Fibroblast growth factor 2 up regulates telomerase activity in neural precursor cells

During brain development, neuronal and glial cells are generated from neural precursors on a precise schedule involving steps of proliferation, fate commitment and differentiation. We report that telomerase activity is highly expressed during embryonic murine cortical neurogenesis and early steps of gliogenesis and progressively decreases thereafter during cortex maturation to be undetectable in the normal adult brain. We evidenced neural precursor cells (NPC) as the principal telomerase-expressing cells in primary cultures from E15 mouse embryo cortices. Their differentiation either in neurons or in glial cells lead to a down regulation of telomerase activity that was directly correlated to the decrease of telomerase core protein (mTERT) mRNA synthesis. Furthermore, we show that FGF2 (fibroblast growth factor 2), one of the main regulators of CNS development, induces a dose-dependant increase of both the proliferation of NPC and telomerase activity in primary cortical cultures without affecting the mTERT mRNA synthesis compared to that of glyceraldehyde-3-phosphate dehydrogenase (mGAPDH). Finally, we evidenced that AZT (3'-azido-2', 3'-dideoxythymidine), known to inhibit telomerase activity, blocks in a dose dependant manner the FGF2-induced proliferation of NPC. Altogether, our results are in favor of an important role of telomerase activity during brain organogenesis. Oncogene (2000).

Na+-dependent high-affinity glutamate transport in macrophages

Excessive accumulation of glutamate in the CNS leads to excitotoxic neuronal damage. However, glutamate clearance is essentially mediated by astrocytes through Na+-dependent high-affinity glutamate transporters (excitatory amino acid transporters (EAATs)). Nevertheless, EAAT function was recently shown to be developmentally restricted in astrocytes and undetectable in mature astrocytes. This suggests a need for other cell types for clearing glutamate in the brain. As blood monocytes infiltrate the CNS in traumatic or inflammatory conditions, we addressed the question of whether macrophages expressed EAATs and were involved in glutamate clearance. We found that macrophages derived from human blood monocytes express both the cystine/glutamate antiporter and EAATs. Kinetic parameters were similar to those determined for neonatal astrocytes and embryonic neurons. Freshly sorted tissue macrophages did not possess EAATs, whereas cultured human spleen macrophages and cultured neonatal murine microglia did. Moreover, blood monocytes did not transport glutamate, but their stimulation with TNF-alpha led to functional transport. This suggests that the acquisition of these transporters by macrophages could be under the control of inflammatory molecules. Also, monocyte-derived macrophages overcame glutamate toxicity in neuron cultures by clearing this molecule. This suggests that brain-infiltrated macrophages and resident microglia may acquire EAATs and, along with astrocytes, regulate extracellular glutamate concentration. Moreover, we showed that EAATs are involved in the regulation of glutathione synthesis by providing intracellular glutamate. These observations thus offer new insight into the role of macrophages in excitotoxicity and in their response to oxidative stress.

Microglial cells respond to amyloidogenic PrP peptide by the production of inflammatory cytokines

The scrapie isoform of the prion protein (PrPres) induces neurodegeneration and gliosis in the central nervous system. These features may be reproduced in vitro on exposure of neuronal and glial cultures to PrPres and the peptide HuPr P106-126. In the present study, we investigated the role of microglial cells and astrocytes in the pathological process by studying their molecular response to PrP 106-126 exposure. PrP 106-126 elicited a specific overproduction of pro-inflammatory cytokines IL1beta and IL6 in microglial cells (but not increased expression of TNFalpha, IL10, and TGFbeta1) and over-expression of GFAP in astrocytes. These effects were strictly dependent on the ability of the peptide to form amyloid fibrils. These data strongly suggest that microglial cells contribute to prion-related neurodegenerative processes by producing proinflammatory cytokines in the brain areas of amyloid PrP deposition.