Loss of the Schizophrenia-linked Furin protein from Drosophila mushroom body neurons results in antipsychotic-reversible habituation deficits.

Habituation is a conserved adaptive process essential for incoming information assessment, which drives behavioral response decrement to recurrent inconsequential stimuli and does not involve sensory adaptation, or fatigue. Although the molecular mechanisms underlying the process are not well understood, habituation has been reported defective in a number of disorders including schizophrenia. We demonstrate that loss of furin1, the Drosophila homolog of a gene whose transcriptional downregulation has been linked to schizophrenia, results in defective habituation to recurrent footshocks in mixed sex populations. The deficit is reversible by transgenic expression of the Drosophila or human Furin in adult α,/ß, mushroom body neurons and by acute oral delivery of the typical antipsychotic Haloperidol and the atypical Clozapine, which are commonly used to treat schizophrenic patients. The results validate the proposed contribution of Furin downregulation in schizophrenia and suggest that defective footshock habituation is a Drosophila protophenotype of the human disorder.SIGNIFICANCE STATEMENTGenome Wide Association Studies have revealed a number of loci linked to Schizophrenia, but most have not been verified experimentally in a relevant behavioral task. Habituation deficits constitute a schizophrenia endophenotype. Drosophila with attenuated expression of the Schizophrenia-linked highly conserved Furin gene present delayed habituation reversible with acute exposure to antipsychotics This strongly suggests that footshock habituation defects constitute a Schizophrenia protophenotype in Drosophila. Furthermore, determination of the neurons whose regulated activity is required for footshock habituation provides a facile metazoan system to expediently validate putative Schizophrenia genes and variants in a well-understood simple brain.

Accessing Olfactory Habituation in Drosophila melanogaster with a T-maze Paradigm.

Habituation is the process whereby perceptual changes alter the value of environmental stimuli, enabling salience filtering. This behavioral response decrement is a form of non-associative learning, where the subject learns about the stimulus and does not involve sensory adaptation, sensory or motor fatigue. The range of behavioral responses in D. melanogaster led to the development of a number of habituation paradigms addressing various sensory modalities. Habituation of osmotactic responses has previously been measured with the Y-maze test and required 30 min of odor exposure. Here, we describe an olfactory habituation assay utilizing the widely used in associative learning paradigms T-maze. Continuous or repetitive odor exposure for 4 min is adequate to attenuate osmotactic responses both to attractive and aversive odors. Importantly, the decreased response conforms to habitation parameters, presenting dishabituation and spontaneous recovery. This assay allows the study of habituation after brief odor exposure, but also discriminates between the two distinct phases of the response, an initial habituation latency period followed by habituation. In addition, the characterization of the neuronal circuits implicated in each phase facilitates further study of the molecular components underlying this process.

Temporally specific engagement of distinct neuronal circuits regulating olfactory habituation in Drosophila.

Habituation is the process that enables salience filtering, precipitating perceptual changes that alter the value of environmental stimuli. To discern the neuronal circuits underlying habituation to brief inconsequential stimuli, we developed a novel olfactory habituation paradigm, identifying two distinct phases of the response that engage distinct neuronal circuits. Responsiveness to the continuous odor stimulus is maintained initially, a phase we term habituation latency and requires Rutabaga Adenylyl-Cyclase-depended neurotransmission from GABAergic Antennal Lobe Interneurons and activation of excitatory Projection Neurons (PNs) and the Mushroom Bodies. In contrast, habituation depends on the inhibitory PNs of the middle Antenno-Cerebral Track, requires inner Antenno-Cerebral Track PN activation and defines a temporally distinct phase. Collectively, our data support the involvement of Lateral Horn excitatory and inhibitory stimulation in habituation. These results provide essential cellular substrates for future analyses of the molecular mechanisms that govern the duration and transition between these distinct temporal habituation phases. Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Drk-mediated signaling to Rho kinase is required for anesthesia-resistant memory in Drosophila.

Anesthesia-resistant memory (ARM) was described decades ago, but the mechanisms that underlie this protein synthesis-independent form of consolidated memory in Drosophila remain poorly understood. Whether the several signaling molecules, receptors, and synaptic proteins currently implicated in ARM operate in one or more pathways and how they function in the process remain unclear. We present evidence that Drk, the Drosophila ortholog of the adaptor protein Grb2, is essential for ARM within adult mushroom body neurons. Significantly, Drk signals engage the Rho kinase Drok, implicating dynamic cytoskeletal changes in ARM, and this is supported by reduced F-actin in the mutants and after pharmacological inhibition of Drok. Interestingly, Drk-Drok signaling appears independent of the function of Radish (Rsh), a protein long implicated in ARM, suggesting that the process involves at least two distinct molecular pathways. Based on these results, we propose that signaling pathways involved in structural plasticity likely underlie this form of translation-independent memory.

Interference of the complex between NCS-1 and Ric8a with phenothiazines regulates synaptic function and is an approach for fragile X syndrome.

The protein complex formed by the Ca2+ sensor neuronal calcium sensor 1 (NCS-1) and the guanine exchange factor protein Ric8a coregulates synapse number and probability of neurotransmitter release, emerging as a potential therapeutic target for diseases affecting synapses, such as fragile X syndrome (FXS), the most common heritable autism disorder. Using crystallographic data and the virtual screening of a chemical library, we identified a set of heterocyclic small molecules as potential inhibitors of the NCS-1/Ric8a interaction. The aminophenothiazine FD44 interferes with NCS-1/Ric8a binding, and it restores normal synapse number and associative learning in a Drosophila FXS model. The synaptic effects elicited by FD44 feeding are consistent with the genetic manipulation of NCS-1. The crystal structure of NCS-1 bound to FD44 and the structure-function studies performed with structurally close analogs explain the FD44 specificity and the mechanism of inhibition, in which the small molecule stabilizes a mobile C-terminal helix inside a hydrophobic crevice of NCS-1 to impede Ric8a interaction. Our study shows the drugability of the NCS-1/Ric8a interface and uncovers a suitable region in NCS-1 for development of additional drugs of potential use on FXS and related synaptic disorders.

Learning and memory deficits consequent to reduction of the fragile X mental retardation protein result from metabotropic glutamate receptor-mediated inhibition of cAMP signaling in Drosophila.

Loss of the RNA-binding fragile X protein [fragile X mental retardation protein (FMRP)] results in a spectrum of cognitive deficits, the fragile X syndrome (FXS), while aging individuals with decreased protein levels present with a subset of these symptoms and tremor. The broad range of behavioral deficits likely reflects the ubiquitous distribution and multiple functions of the protein. FMRP loss is expected to affect multiple neuronal proteins and intracellular signaling pathways, whose identity and interactions are essential in understanding and ameliorating FXS symptoms. We used heterozygous mutants and targeted RNA interference-mediated abrogation in Drosophila to uncover molecular pathways affected by FMRP reduction. We present evidence that FMRP loss results in excess metabotropic glutamate receptor (mGluR) activity, attributable at least in part to elevation of the protein in affected neurons. Using high-resolution behavioral, genetic, and biochemical analyses, we present evidence that excess mGluR upon FMRP attenuation is linked to the cAMP decrement reported in patients and models, and underlies olfactory associative learning and memory deficits. Furthermore, our data indicate positive transcriptional regulation of the fly fmr1 gene by cAMP, via protein kinase A, likely through the transcription factor CREB. Because the human Fmr1 gene also contains CREB binding sites, the interaction of mGluR excess and cAMP signaling defects we present suggests novel combinatorial pharmaceutical approaches to symptom amelioration upon FMRP attenuation.