Aggregation Inhibitors / Toxicity inhibition for alpha-synuclein

Dramatically different methods and approaches have been described that can suppress a-syn aggregation and provide protection from toxicity in model systems. Blocking the formation of aggregated a-syn structures by inhibition with short synthetic peptides represents one avenue. Peptides derived from the N-terminal amino acid sequence (1 to 15) of b-synuclein display neuroprotective activity. This peptide
sequence may be an antiaggregation factor for a-syn toxicity induced by oxidative stress. When peptide fragments are derived from the a-syn protein itself, some sequences demonstrate the propensity to inhibit fibril formation and toxicity. Overall, the peptides may act as b-sheet breakers; the shortest peptide, RGAVVTGR-amide, retains the ability to inhibit a-syn aggregation. In culture systems, a cell-permeable peptide inhibitor of a-syn aggregation inhibited DNAdamage
induced by Fe2þ in neuronal cells expressing mutant human a-syn (A53T). Therapeutic delivery of b-sheet–breaking peptides to the brain of PD-affected individuals represents a monumental challenge, but small peptides may demonstrate
proof of principle in model systems that correlate a-syn aggregation
with toxicity.
b-Synuclein (a 134-amino-acid protein) also prevents a-syn aggregation in vitro and in double-transgenic mice. Acting as chaperones, b- and g-synuclein both reduce the rate of a-syn fibrillation and aggregation. Hsp70 is a potent protein chaperone and refolding complex also known to inhibit aggregation and fibril formation by preferential binding to prefibrillar species. Hsp70 likewise potently reduces a-syn self-interaction in bimolecular fluorescence complementation assays. Other chaperones
that may play critical roles in anti–a-syn aggregation include torsinA , Hsp40, and aB-crystallin.
Broad therapeutic delivery of protective proteins that target a-syn toxicity and aggregation throughout the brain requires huge advances in current gene-therapy technology, but in the meantime, proteins that protect from a-syn toxicity in relevant model systems will help delineate pathogenesis and provide
additional therapeutic targets that in turn may be amenable small-molecule modification.
Human single-chain antibody fragments (scFv) that bind asyn inhibit toxicity and formation of a-syn–positive fibrils. scFv molecules can bind specifically to an oligomeric form of a-syn and prevent aggregation and interaction with the cell membrane, thereby reducing membrane damage and pore formation. Like peptides with an affinity for b-sheets and chaperones with an affinity for unfolded or aggregated proteins, the isolated scFvs ideally bind only to the toxic oligomeric
species in the target protein while avoiding the problem of interaction with the potentially benign and abundant natively unfolded a-syn protein. Intrabody therapy, as with most recombinant protein approaches in therapeutics, introduces
an additional set of difficult technical challenges in the clinic beyond the question of target relevance.
Inflammation and microglial activation coincide with neurodegeneration in PD and in many models of the disease. Microglia inflammation inhibitors and antiinflammatory approaches are under investigation for preventing a-syn toxicity as well as to suppress neuroinflammation in PD. Suppressive action by NSAIDs on dopamine quinone
formation by interaction of a-syn with microglia and astrocytes either may arrest or effectively slow neurodegeneration. a-Syn mutations may induce a proinflammatory
phenotype in both microglia and astrocytes, indicating the involvement of cell-surface receptors for both microglia and astrocytes. Antagonists for these putative cell surface receptors (microglial and astrocyte), as well as those
for other molecules that regulate microglial activation, including MMP-3, CD40L, CCL2, and other chemokines could constitute novel targets for therapeutic intervention.
In another study in which a commercially available compound library was screened, it was found that dopamine and other catecholamines interacted with a-syn protofibrils and inhibited the fibrilization process. When antioxidants like sodium metabisulfite were added, the process was reversed, suggesting that fibril inhibition, protofibril accumulation, and monomer modification is a sequel to covalent modification
by the dopamine-derived orthoquinone. Other compounds with antioxidative properties such as flavonoid baicalein and some antibiotics like rifampicin are
also able to inhibit a-syn fibrillation in vitro and further disaggregate
preformed fibrils and soluble oligomers. PD therapeutic agents such as selegiline, dopamine, pergolide, and bromocriptine dose-dependently inhibit the formation of asyn
fibrils and also destabilize the preformed a-syn fibrils. The potency of these compounds ranks as follows: selegiline¼dopamine>pergolide>bromocriptine. In short,
small molecules exist that likely modify a-syn structure in cells. Targeted screens that elucidate molecules with drug-like properties and demonstrate efficacy in relevant model systems will ultimately test the role of aggregation and a-syn
protofibril formation in disease pathogenesis.