Protein misfolding is the molecular basis for several human diseases. How the primary amino acid sequence triggers misfolding and determines the benign or toxic character of the misfolded protein remains largely obscure. Among proteins that misfold, polyglutamine (polyQ) expansion proteins provide an interesting case: Each causes a distinct neurodegenerative disease that selectively affects different neurons. However, all are broadly expressed and most become toxic when the glutamine expansion exceeds Ϸ39 glutamine residues. The disease-causing polyQ expansion proteins differ profoundly in the amino acids flanking the polyQ region. We therefore hypothesized that these flanking sequences influence the specific toxic character of each polyQ expansion protein. Using a yeast model, we find that sequences flanking the polyQ region of human huntingtin exon I can convert a benign protein to a toxic species and vice versa. Further, we observe that flanking sequences can direct polyQ misfolding to at least two morphologically distinct types of polyQ aggregates. Very tight aggregates always are benign, whereas amorphous aggregates can be toxic. We thereby establish a previously undescribed systematic characterization of the influence of flanking amino acid sequences on polyQ toxicity.

SUMMARY Behavior cannot be predicted from a “connectome,” because the brain contains a chemical “map” of neuromodulation superimposed upon its synaptic connectivity map. Neuromodulation changes how neural circuits process information in different states, such as hunger or arousal. Here we describe a novel, genetically based method to map, in an unbiased and brain-wide manner, sites of neuromodulation under different conditions in the Drosophila brain. This method, and genetic perturbations, reveal that the well-known effect of hunger to enhance behavioral sensitivity to sugar is mediated, at least in part, by the release of dopamine onto primary gustatory sensory neurons, which enhances sugar-evoked calcium influx. These data reinforce the concept that sensory neurons constitute an important locus for state-dependent gain-control of behavior, and introduce a new methodology that can be extended to other neuromodulators and model organisms.

Several neurodegenerative diseases are associated with the toxicity of misfolded proteins. This toxicity must arise from a combination of the amino acid sequences of the misfolded proteins and their interactions with other factors in their environment. A particularly compelling example of how profoundly these intramolecular and intermolecular factors can modulate the toxicity of a misfolded protein is provided by the polyglutamine (polyQ) diseases. All of these disorders are caused by glutamine expansions in proteins that are broadly expressed, yet the nature of the proteins that harbor the glutamine expansions and the particular pathologies they produce are very different. We find, using a yeast model, that amino acid sequences that modulate polyQ toxicity in cis can also do so in trans. Furthermore, the prion conformation of the yeast protein Rnq1 and the level of expression of a suite of other glutamine-rich proteins profoundly affect polyQ toxicity. They can convert polyQ expansion proteins from toxic to benign and vice versa. Our work presents a paradigm for how a complex, dynamic interplay between intramolecular features of polyQ proteins and intermolecular factors in the cellular environment might determine the unique pathobiologies of polyQ expansion proteins.huntingtin ͉ Huntington's disease ͉ protein misfolding ͉ yeast A variety of human diseases are characterized by the accumulation of aggregated, misfolded proteins (1, 2). In some cases, such as cystic fibrosis, disease is caused by the loss of a vital protein function due simply to this misfolding and aggregation. In other cases, however, protein misfolding creates novel, toxic properties. For example, in Alzheimer's disease, Parkinson's disease, and the polyglutamine (polyQ) disorders, misfolded proteins disrupt proper cellular function and cause cytotoxicity (3,4). In all of these diseases the amino acid sequences of the particular misfolded protein and their interactions determined by its specific environment must govern toxicity. Yet, despite intense research, we know little about these intramolecular and intermolecular factors. Without identifying and understanding these factors at a molecular level it will be difficult to devise the most effective therapeutic strategies to treat protein misfolding diseases.The polyQ diseases present a promising starting point to study this very general problem. In all of these diseases polyQ expansions constitute the molecular basis of disease (5). However, the disease pathologies (including the cell types most strongly affected) and the individual proteins that bear the disease-causing polyQ expansions are very distinct (5). Consequently, the amino acids flanking the polyQ region and the sets of cellular factors that specifically interact with each polyQ expansion protein are different. Combinations of these factors must account for their different pathobiologies. Recent studies have started to identify protein-protein and genetic interaction networks for polyQ proteins (6-8), but the current data do not yet ...

Due to a copyediting error, in the sentence below from the Results section of the above article, the number ''15'' was inadvertently replaced with ''150.'' A corrected version of the article now appears online.''The dynamic range of this reporter (2-to 15-fold; Figures 2B 1 -2B 4 ) is similar to that of the best currently available genetically encoded calcium indicators (GECIs) (Tian et al., 2009), although the signal-to-noise ratio (SNR; c.a. 4) is lower (see Extended Experimental Procedures).''

SUMMARY In Drosophila, the four inner photoreceptor neurons exhibit overlapping but distinct spectral sensitivities and mediate behaviors that reflect spectral preference. We have developed a genetic strategy, Tango-Trace, that has permitted the identification of the connections of the four chromatic photoreceptors. Each of the four stochastically distributed chromatic photoreceptor subtypes make distinct connections in the medulla with four different TmY cells. Moreover, each class of TmY cells forms a retinotopic map in both the medulla and the lobula complex, generating four overlapping topographic maps that could carry different color information. Thus, the four inner photoreceptors transmit spectral information through distinct channels that may converge in both the medulla and lobula complex. These projections could provide an anatomic basis for color vision and may relay information about color to motion sensitive areas. Moreover, the Tango-Trace strategy we have employed may be applied more generally to identify neural circuits in the fly brain.