A critical appraisal of the pathogenic protein spread hypothesis of neurodegeneration

Abstract

There has been an explosion in the number of papers discussing the hypothesis of 'pathogenic spread' in neurodegenerative disease - the idea that abnormal forms of disease-associated proteins, such as tau or α-synuclein, physically move from neuron to neuron to induce disease progression. However, whether inter-neuronal spread of protein aggregates actually occurs in humans and, if so, whether it causes symptom onset remain uncertain. Even if pathogenic spread is proven in humans, it is unclear how much this would alter the specific therapeutic approaches that are in development. A critical appraisal of this increasingly popular hypothesis thus seems both important and timely.

Figure 1. The pathogenic spread and selective vulnerability hypotheses

There are at least two possible explanations for how the localization of aggregates in the brain might change as neurodegenerative diseases progress. a| According to the pathogenic spread hypothesis aggregates generated in one brain region physically move from neuron to neuron and thus spread into connected brain regions. b| The selective vulnerability hypothesis suggests that, in response to certain adverse conditions (such as external stress), protein aggregation is initiated in a subset of neurons that are particularly vulnerable to the adverse stimuli. Protein aggregates first appear in the cells most susceptible to the adverse conditions, and with time emerge in less susceptible cells. This hypothesis also supports the idea that disease pathogenesis may spread trans-synaptically: however, it suggests that this is mediated by the spread of diffusible metabolic factors that result in the transduction of the effects of the adverse conditions to a neighbouring neuron, rather than a direct physical transfer of protein aggregates. It is important to note that these 2 possibilities are not mutually exclusive and that a combination of both hypotheses may occur.

Figure 2. Possible mechanisms for inter-neuronal transfer of proteins

In the figure, two neurons are synaptically connected, with illustrative synapses shown in magnified views. A third neuron (lower center) is near to the other neurons but not synaptically connected. Neither α−synuclein nor tau contain the signal sequences necessary for conventional secretion, so their release must occur by a non-classical mechanism. Over the past decade, two non-classical mechanisms for inter-cellular communication have emerged: the secretion of small vesicles called exosomes (1), and the formation of thin membranous bridges termed tunneling nanotubes (TNTs) (2). Exosomes could potentially travel across synapses or longer distances and facilitate transfer of proteins to other cell types. Similarly, TNTs facilitate communication between neural and non-neural cells, often over long distances. Interestingly, both exosomes and TNTs have been implicated in the movement of infectious PrP in experimental models. If cell-to-cell transfer of non-PrP neurodegenerative disease-related proteins occurs via exosomes or TNTs, then the proteins inside these structures are unlikely to be fully accessible to antibodies used in immunotherapy (see Figure. 3). A third possible mechanism (3) involves the release and uptake of the naked protein. α−synuclein is present in pre-synaptic endings^, , and tau is present in post-synaptic elements^, . During neurotransmission, it is possible that small amounts of either protein could leak between the pre- and post-synapse. The more abundant a neuronal protein is, the more likely this is to occur (both α−synuclein and tau are highly abundant)^, . Finally, an obvious but little discussed possibility (4) is that proteins are released as a secondary effect of synaptic or cellular compromise. Little is known about the cell biological mechanisms by which pathogenic proteins that are released are taken up by neurons and how they encounter the cognate endogenous protein that they are proposed to template.

Figure 3. Strategies for targeting disease-associated neural protein aggregates

a| The steady state levels of all proteins are controlled by their rates of production and degradation. Above a certain critical concentration, monomers can self-associate to form abnormal dimers, trimers, larger oligomers and insoluble aggregates. Consequently, reducing the levels of monomers by inhibiting their production (1) or stimulating their degradation (2) should decrease formation of pathogenic oligomers and larger aggregates. Agents that bind to and stabilize the native protein (3) should prevent abnormal oligomerization and allow for the natural removal of the protein by the brain’s degradative machinery. In this regard, an agent which stabilizes the native structure of the transthyretin (TTR) tetramer, tafamidis, has been approved for treating TTR amyloidosis, and an analogous approach may be feasible for the native α-synuclein tetramer. Conversely, agents capable of disrupting abnormal oligomers (4) should reduce their concentration and may prevent formation of larger aggregates such as fibrils. Antibodies or small molecules capable of binding to various abnormal assemblies (5) could neutralize the activity of oligomers and/or facilitate the clearance of deposited aggregates. In the case of binding by antibodies, this may include uptake of the complexes by microglia and/or their transport out of the brain. Peripherally administered antibodies should also be effective in the case of potential “pathogenic spread” from the blood or lymphatic system to the CNS. For simplicity, we refer to the native assembly state of neurodegeneration-associated proteins as monomer; however, there is growing evidence that αSyn normally exists as a tetramer, in which case the first step in the pathogenic aggregation process would be tetramer disassembly to excess free monomers inside neurons. All 5 therapeutic approaches summarized here could be applicable to both extracellular and intracellular pathogenic proteins. B| If intracellular aggregation requires direct movement of aggregates from one neuron to another, then two additional approaches would be to inhibit the release of protein seeds (6) and to inhibit their re-uptake (7). If these processes occur via exosomes or tunneling nanotubes (Fig. 1), they may not be accessible to extracellular agents such as antibodies, and therefore would require new therapeutic strategies.