Progress and Prospects in Parkinson's Research/Magazine Section/3 hits in understanding pathogenesis

It can't be often that a piece of research work is significant in three important ways. The work that Prof Virginia Lee of the University of Pennsylvania described at the World Parkinson Congress in Montreal in October 2013 fitted this category. The title of her presentation, Propagation of the neurodegenerative process in PD and the Prion-like hypothesis, appeared merely to address the question of whether alpha-synuclein pathology spreads by a prion-like process. It did indeed address that. But there were two other results of the work that also represent significant steps forward.

Evidence supporting the prion-like process
Early in this century Braak et al noted that the characteristic pathology of Parkinson's which involves the aggregation and deposition within neurons of the protein alpha-synuclein appeared to spread up from its initial appearance in the brain stem to other parts of the brain in a more or less predictable pattern. They surmised that some unknown infective agent was carrying the disease process in a retrograde fashion through the axons connecting the neurons in the different locations that were progressively affected.

A more recent view is that abnormally folded alpha-synuclein simply spreads from cell to cell in a prion-like way, 'seeding' unaffected protein molecules causing them to associate into insoluble fibrils. This process appears to be common to many neurological conditions as well as Parkinson's.

The work of the Virginia Lee team has reinforced confidence in the prion-like hypothesis by leveraging the phenomenon within the experimental context to explore the seeding process further.

Different strains of aggregated protein
The study published by the Lee team in July 2013 demonstrated that distinct synthetic alpha-synuclein strains with different seeding properties can be generated in vitro.

Alpha-synuclein is prone to aggregation which can be induced in many ways. In this series of experiments, fibrils from a sample of a solution of alpha-synuclein monomer which had been induced to begin to aggregate were taken and used to 'seed' another solution of monomer. The aggregation proceeded much more rapidly the second time than in the unseeded solution.

Fibrils (now referred to as pffs or pre-formed fibrils) from this second solution were then taken to seed a third solution and the process repeated as many as 6 or more times. Pffs from each of these 'passages' were introduced to neural cell cultures taken from mouse brains. The significant discovery was that pre-formed fibrils from the later passages affected the cell pathology in a different way from those taken from the earlier passages. Seeds from passage 1 (P1) resulted in profuse alpha-synuclein inclusions while those from subsequent passages led to diminishing alpha-synuclein pathology and an increasing amount of neuritic tau pathology. By P6 or P7 there was abundant tau pathology. Tau is the protein which aggregates to form plaques and neurofibrillary tangles in Alzheimer's disease but is also deposited in some cases of Parkinson's. The differing behaviours of the pffs from the earlier and late passes pointed to the existence of two strains of abnormal alpha-synuclein, A and B, with strain B being a highly effective template for the aggregation of tau. Subsequent detailed analysis showed that strains A and B differ in their conformation - i.e. in the way the two varieties of the same protein molecule are folded.

Earlier experiments in the series used abnormal, truncated alpha-synuclein from genetically modified mice but later ones used fibrils from full-length alpha-synuclein from non-transgenic mice (known as 'wild-type' mice). In both cases the seeding effects were assessed in neural cell cultures from non-transgenic mice and were found to be very similar. This showed that the phenomenon was not merely due the particular nature of aberrant protein nor the high level of expression in transgenic mice. The two strains of protein pffs could be produced from wild-type monomer and were effective in seeding aggregation in cell cultures from wild-type mice.

A later series of experiments were conducted in vivo. Pre-formed fibrils were injected into the hypocampal areas of the brains of wild-type and transgenic mice and broadly the same differential strain-dependent patterns of seeding-induced pathology were observed to occur in both. (See the original paper for subtleties that we can't go into here.)

These results are consistent with the occurrence of differing variations of Parkinson's, some displaying tau pathology and some not. The different variations of the disease could be due to the development of different strains of aberrant alpha-synuclein. Furthermore, other as yet undiscovered strains could be at the root of synucleinopahies affecting other areas of the brain such as in MSA where glial cells in the striatum are affected.

A better mouse model
This is the third level of significance. Up to now, animal models of Parkinson's have been inadequate. They are either lesion models in which, for instance, a toxin such as MPTP or 6-hydroxydopamine (6-OHDA) is administered so as to damage cells in the brain or they are transgenic mouse models in which alpha-synuclein is over-expressed and insoluble protein aggregates are formed. In the first case there is dopamine neuron loss and parkinsonian symptoms but no Lewy bodies and in the second there are Lewy bodies but no dopamine loss and consequently no symptoms.

While investigating the prion-like spread of alpha-synuclein pathology, the Lee team came up with a mouse model displaying the characteristic features of idiopathic Parkinson's: Lewy-like deposits, dopamine neuron depletion and Parkinson-like symptoms. This was through a series of experiments investigating the spread of the pathology following the injection of pffs into certain specific structures in the brain known to be affected in Parkinson's.

First, when the injection side was the dorsal striatum, the pathology was found to spread to the other centres to which this has axonal connections. But in addition the pathology reached the olfactory bulb which has no direct connection to the striatum suggesting that trans-synaptic transfer of pathogenic alpha-synucelin occurs. This spread of pathology mimics very well the pattern observed in Parkinson's. The pathology also eventually reached the hemisphere opposite to the one injected probably via the amygdala which is again reminiscent of Parkinson's where symptoms start on one side and eventually affect the other.

Second, injection of pffs either in the striatum or directly into the substantia nigra pars compactor, produced deposits of aggregated alpha-synuclein in the dopaminergic neurons. Over time the observable level of alpha-synuclein declined and this was because there was degeneration and loss of dopaminergic cells containing them again mimicking Parkinson's.

The loss of dopaminergic neurons led to the impairment of movement which was observed through the standard 'rotarod' and 'wire hang' tests.

In her talk at the World Parkinson Congress Prof Virginia Lee was keen to emphasise that this was a very easy-to-use animal model. It involved no genetic engineering and therefore had the potential to be an excellent model for drug discovery research especially for the preponderant, non-genetic forms of Parkinson's.

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