Participants agreed that protein misfolding occurs with striking similarity among different neurodegenerative diseases, but it remains unclear whether protein misfolding precedes or is secondary to synaptic and axonal dysfunction. It will be important to place protein misfolding and proteasomal/lysosomal degradation into the cascade of AD pathology. On the basic research side, Susan Lindquist, Whitehead Institute, Cambridge, discussed surprising data suggesting that prion conformation of some yeast proteins changes the protein's function and the cell's metabolism, but is not inherently toxic. Indeed, prion formation per se is advantageous for yeast. This raises the question of whether some aggregation-prone proteins have an alternative, self-perpetuating conformation that serves an unknown physiological role, perhaps in synaptic function. One example is shown in yeast/aplysia (see related news story). Given the strong evolutionary conservation in the way these proteins behave, it is worth examining this question in higher organisms. Participants discussed the concept of natively unfolded proteins, which include tau, α-synuclein, and Aβ.
Why are aggregates toxic, and which ones are toxic? There was consensus that large aggregates form to protect the cell from more toxic, smaller ones. Large fibrillar aggregates are almost crystalline in their degree of organization. They bury within them the noxious side chains and species that stick out from intermediate species in an almost combinatorial array of different surface groups that have not evolved to be exposed. The slew of oligomeric species is truly different from fibrillar forms in that they are more exposed, less stable, and easier to degrade. Some intermediate species are inherently toxic. Plaques later cause a secondary, different toxicity, for example by attracting and distorting neurites.
Christopher Dobson of University of Cambridge, UK, suggested that oligomer toxicity has evolved as a way of removing a cell burdened by intermediate species before large amounts can accumulate and infect other cells. Death of an oligomer-laden cell might be a protective response, analogous to death of virus-infected cells. The toxic mechanism of Aβ oligomers in neurons must be determined.
The role of chaperones deserves further study. These proteins may act in opposing ways in neurodegenerative diseases versus cancer, whereby increasing the chaperone balance might protect against the former but predispose to the latter. This is a caveat against drug development based on chaperones. The only risk factor for AD, ApoE, is a chaperone that influences the dynamics of Aβ deposition versus removal. Its mechanism needs more study.
A useful tool for the study of chaperone biology and mechanisms would be a catalog of chaperones in other species, especially in extreme organisms that have evolved chaperones capable of stabilizing proteins in extreme temperatures, osmotic stress, etc.
Is protein misfolding the primary event in AD? Are tau and amyloid changes visible examples of a broader process? How to test these questions?
This research direction arises from work showing that protein folding is a stochastic process with a given error rate. A small amount of misfolding is even necessary so that proteins can be degraded and displayed to induce immune tolerance against self. Under normal conditions misfolded proteins are swept away by degradation systems, preventing aggregation. When this system starts to fail, damage first appears in the most vulnerable cells. A person can have a kilogram of lysozyme in body and still be alive, while a gram of protein aggregated in brain is lethal. Why? This area is wide open for discovery.
List of Final Recommendations
1. Make better mouse models. Create strains with only subtle overexpression under endogenous promoter and authentic spatiotemporal regulation, such as YAC. Recreate humanized APP rat unavailable from Cephalon Inc.
2. Develop an arsenal of imaging markers for pathologies other than amyloid, probes for tau, α-synuclein. Develop imaging probes that report on functional state of synapse, not just structure of synapse.
3. Test Goldstein's hypothesis. Develop probes for imaging axonal flow in animal models and humans. Study peripheral nervous system defects in axonal transport, search for peripheral markers of that.
4. Develop small-molecule probes that cross BBB and identify particular protein aggregates in brain non-invasively. Does binding predict disease?
5. Determine normal function of APP, PS. Study signaling role of Aβ as lead toward toxicity mechanism.
6. With cell biologists: Elucidate role of APP, Aβ, and oligomers in synaptogenesis and synapse function. Elucidate molecular mechanism of synapse loss and look for synaptic defects directly caused by APP/PS mutations.
7. Better cell biology. Develop models of neurons in 3D matrices as more realistic setting to study Aβ toxicity.
8. Understand process of Aβ aggregation into oligomers. Expand work on good cellular systems for that, such as Lindquist's.
9. Next generation of basic scientists: Have each PI bring a postdoc to workshop.
10. Develop better cognitive assessments of early AD.
11. Develop teams of statisticians and geneticists to follow disease in clinic.
12. Build framework for computational model of the disease process. Support computational modeling of small-molecule oligomer inhibitors.
13. Establish centenarian gene bank to look for protective APP/PS SNPs in sharp old old. In parallel, run unbiased screen for genetic/expression differences in them.
14. Better exploration of interaction between astrocytes and neurons. Astrocyte populations are highly diverse. Field is still at descriptive stage; move beyond that.
15. Run FISH on several hundred sporadic AD cases to look for APP duplication in peripheral cells, like the α-synuclein triplication causing PD, or Down's.
16. Run massive gene expression and proteomic screens to ascertain whether there are other players that we don't know about yet.
17. Organize quantitative information so it is widely available. Find data sharing mechanisms, such as the fMRI Data Center.
18. Put knowledge pieces together by establishing online AD pathway model. Create an online matrix of mouse, yeast, fly, worm, cell-based model systems.
19. Facilitate establishment of, and access to, DNA collections for genetics studies to identify additional players.
20. Bring structural biologists to the field.
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