Blocking the formation of the disease associated protein aggregates represents a major target for therapeutic intervention and as was evident from several presentations at the conference, real progress is beginning to be made in the search for small molecule inhibitors—be they natural or synthetic in origin—which block the formation of toxic aggregates or which abrogate their toxicity.
One of the challenges is to develop a suitable assay for candidate compounds that has sufficiently high throughput to allow the screening of the ever-increasing chemical libraries. Erich Wanker (Berlin) and his colleagues have used a cell free screen to identify a number of small molecules that inhibit polyQ/htt aggregation. The ability of these compounds to reduce fibril formation and suppress toxicity at the organismal level was confirmed using yeast, Drosophila and mammalian cell based assays. Intriguingly some of the compounds examined promoted the formation of off pathway non toxic oligomers rather than blocking protein aggregation per se. Among such compounds identified was epigallocatechin gallate (EGCG) a component of green tea and the htt oligomers formed in the presence of EGCG are not toxic to cultured cells. Inducing the conversion of amyloidogenic proteins into off pathway and non toxic oligomeric structures may therefore represent a promising new therapeutic strategy to prevent or at least slow down the pathogenesis associated with amyloidoses.
While the actual target for EGCG and other compounds remains to be defined, Marc Blondel (Brest) described the mechanism of action of a group of antiprion drugs originally identified by his group using both yeast and mammalian cell based assays. The cellular target of two of the most potent antiprion compounds they discovered, 6 aminophenanthridine and another drug already in the clinic for other applications, appear to target a chaperone activity associated with the heavy subunit of the ribosome (50S). Both compounds eliminate fungal and mammalian prions providing strong evidence for the universality of the process by which prions are propagated in fungi and humans.
Several other approaches to screen for new therapeutic agents active against one or more conformational diseases were also described. Based on the observation that the peptide NFGAIL loses its assembly propensity when the residue F is changed to Y or aliphatic amino acids, but not W, Ehud Gazit (Israel) suggested that the packing of aromatic residues is critical for the initiation of assembly and therefore should be targeted for anti assembly therapeutic strategies. Based on these findings, a novel inhibitor was found to restore cognitive performance in a mouse AD model. Similarly, and following the observation that a peptide derived from IAPP (IAPP GI) inhibits the cytotoxic self assembly of both Aβ and IAPP polypeptides, Aphrodite Kapurniotu (Aachen) suggested that such synthetic peptides may constitute promising therapeutic strategies targeting both AD and type 2 diabetes.
Jeff Kelly (La Jolla) proposed a novel way of treating type 1 Gaucher disease that is associated with the loss of function of a glucocerebrosidase. Kelly's approach was to identify inhibitors of this enzyme that would stabilize a significant proportion of the enzyme molecules leading to the accumulation of the enzyme inhibitor complex within the cells. At very high concentrations of the enzyme inhibitor complex, a fraction of the enzyme is found unbound to its inhibitor because of the dissociation constant and this leads to a basal level of enzymatic activity and hence survival.
In addition to identifying compounds, cellular targets are also being identified to facilitate rational drug design strategies. Paul Muchowski (San Francisco) described his group's use of a yeast based screen to identify genes associated with the toxicity of a mutant frag- ment of htt and this has led to the finding that deletion of the yeast gene BNA4, that encodes kynurenine 3 mono oxygenase (KMO), alleviates such toxicity. KMO is a mitochondrial enzyme implicated in the kynureinine pathway of tryptophan degradation and is also found exclusively in the microglia in the central nervous system (CNS). Muchowski suggested that the mutant htt may induce a transcriptional defect that activates the kynureinine pathway in the CNS and that this contributes to the neurodegeneration seen in HD patients. Human KMO therefore represents a novel ‘druggable’ target since its inhibition, as already shown by Muchowski's group, has significant beneficial effects in a mouse model of HD.