Tastekin, I. | bioRxiv Science

bioRxiv2022

marmite defines a novel conserved neuropeptide family mediating nutritional homeostasis

Neuropeptides play a key role in regulating physiology and behavior, including feeding. While animals modify their food choices to respond to the lack of specific nutrients, the mechanisms mediating nutrient-specific appetites remain unclear. Here, we identified marmite (mmt), a previously uncharacterized Drosophila melanogaster gene encoding a secreted peptide that controls feeding decisions. We show that both mmt mutants and neuronal knockdown of mmt specifically increased the intake of proteinaceous food, whereas neuronal mmt overexpression reduced protein appetite. mmt expression is also higher in animals maintained on amino acid rich food, suggesting that mmt encodes a protein-specific satiety signal. Mmt is expressed in a small number of neurons in the adult nervous system, with a single pair of neurons modulating protein appetite. Finally, sequence and phylogenetic analysis showed that mmt is part of an ancient and conserved family of neuropeptides, including the poorly understood vertebrate neuropeptides B and W genes. Functional experiments showed that mmt and vertebrate NPB and NPW modulate food intake in both flies and mice. Therefore, we discovered an ancient family of neuropeptides involved in controlling feeding across phyla.

neuroscience↗

bioRxiv2022

An essential experimental control for functional connectivity mapping with optogenetics

To establish functional connectivity between two candidate neurons that might form a circuit element, a common approach is to activate an optogenetic tool such as Chrimson in the candidate pre-synaptic neuron and monitor fluorescence of the calcium-sensitive indicator GCaMP in a candidate post-synaptic neuron. While performing such experiments, we found that low levels of leaky Chrimson expression can lead to strong artifactual GCaMP signals in presumptive postsynaptic neurons even when Chrimson is not intentionally expressed in any particular neurons. Withholding all-trans retinal, the chromophore required as a co-factor for Chrimson response to light, eliminates GCaMP signal but does not provide an experimental control for leaky Chrimson expression. Leaky Chrimson expression appears to be an inherent feature of current Chrimson transgenes, since artifactual connectivity was detected with Chrimson transgenes integrated into three different genomic locations (two insertions tested in larvae; a third insertion tested in the adult fly). These false-positive signals may complicate the interpretation of functional connectivity experiments. We illustrate how a no-Gal4 negative control improves interpretability of functional connectivity assays. We also propose a simple but effective procedure to identify experimental conditions that minimize potentially incorrect interpretations caused by leaky Chrimson expression.

neuroscience↗