The formation of polyglutamine-containing aggregates and inclusions are hallmarks of pathogenesis in Huntington's disease that can be recapitulated in model systems. Although the contribution of inclusions to pathogenesis is unclear, cell-based assays can be used to screen for chemical compounds that affect aggregation and may provide therapeutic benefit. We have developed inducible PC12 cell-culture models to screen for loss of visible aggregates. To test the validity of this approach, compounds that inhibit aggregation in the PC12 cell-based screen were tested in a Drosophila model of polyglutamine-repeat disease. The disruption of aggregation in PC12 cells strongly correlates with suppression of neuronal degeneration in Drosophila. Thus, the engineered PC12 cells coupled with the Drosophila model provide a rapid and effective method to screen and validate compounds.

The crystal structure of the pheromone Er-I from the unicellular eukaryotic organism Euplotes raikovi was determined at 1.6 A resolution and refined to a crystallographic R factor of 19.9%. In the tightly packed crystal, two extensive intermolecular helix-helix interactions arrange the Er-i molecules into layers. Since the putative receptor of the pheromone is a membrane-bound protein, whose extracellular C-terminal domain is identical in amino acid sequence to the soluble pheromone, the interactions found in the crystal may mimic the pheromone-receptor interactions as they occur on a cell surface. Based on this, we propose a model for the interaction between soluble pheromone molecules and their receptors. In this model, strong pheromone-receptor binding emerges as a consequence of the cooperative utilization of several weak interactions. The model offers an explanation for the results of binding studies and may also explain the adhesion between cells that occurs during mating.Pheromones from the ciliated protozoan Euplotes raikovi are proteins of 37-40 amino acids that function both as growth factors and as signaling molecules in cellular adhesion during mating (1-3). Over a dozen different cell types of E. raikovi have been identified based on their ability to form mating pairs (1). In the presence of only one cell type, the homologous secreted Er (Er-x = E. raikovi pheromone of type x) molecules stimulate growth of the cells by binding to cell-surface receptors in an autocrine fashion. In the presence of two different cell types or just one cell type to which heterologous pheromone has been added, Er molecules also stimulate cell adhesion between mating pair partners presumably by binding to the same cell-surface receptors (2, 3) in a paracrine manner. The putative receptors of the pheromones are membranebound proteins whose extracellular C-terminal domain is identical in amino acid sequence to the soluble pheromones (4). They arise by alternate splicing of the transcripts of the same gene that carries the information for the soluble pheromone (4).Different cell types can also be distinguished by the amino acid sequence of their pheromones. Sequences from nine different cell types have been determined (5-8), yielding seven unique sequences with pairwise sequence identities ranging from about 25% to 95%. Only seven residues are conserved among all sequences; these are the N-terminal aspartic acid and six cysteines that are involved in formation of three disulfide bridges (9).The NMR structures of three of the pheromones (Er-1, Er-2, and Er-10) have been determined and compared to each other (10-13). They revealed the three-helical bundle fold of the proteins and the orientations for about two-thirds of the side chains. The crystal structure reported here offers a detailed picture of the monomer and reveals two types of interactions between molecules that provide the basis for a model for receptor recognition, cell adhesion, and signaling. (14) is 1.53 A3/Da and the solvent content of the crystals is ...