Lactate is an energy substrate and intercellular signaling molecule with multiple bodily functions. Lactate has physiological roles in neurogenesis, axon integrity, memory consolidation, immune response, exercise, adipose tissue lipolysis, etc, and is involved in inflammation, cancer and neurodegeneration. The FRET lactate indicator Laconic has been instrumental in the discovery of mechanisms involved in neurometabolic coupling, and has advanced the understanding of lactate transport, glycolysis and mitochondrial physiology. However, the low fluorescent response and the complex saturation kinetics of Laconic limit its use for high-throughput screening and quantitation. Using the bacterial periplasmic binding protein TTHA0766 from Thermus thermophilus, we have now developed the first single-fluorophore indicator for lactate. The sensor exhibited an intensiometric fluorescence increase of ΔF/F0 3.0 and a single binding site with a KD of 293 micromolar. The fluorescence is not affected by other monocarboxylates or pH. However, it is sensitive to Ca2+ in the nanomolar range. Targeting of the sensor to the endoplasmic reticulum revealed that this organelle presents a high permeability for lactate. The functionality of the sensor in living tissue is demonstrated in the brain of Drosophila melanogaster larvae. This indicator, which we have termed CanlonicSF, is well suited to explore lactate dynamics in environments with micromolar Ca2+ or higher, such as the endoplasmic reticulum and the extracellular space.

Lactate is an energy substrate and an intercellular signal, which can be monitored in intact cells with the genetically encoded FRET indicator Laconic. However, the structural complexity, need for sophisticated equipment, and relatively small fluorescent change limit the use of FRET indicators for subcellular targeting and development of high-throughput screening methodologies. Using the bacterial periplasmic binding protein TTHA0766 from Thermus thermophilus, we have now developed a singlefluorophore indicator for lactate, CanlonicSF. This indicator exhibits a maximal fluorescence change of 200% and a K D of ∼300 μM. The fluorescence is not affected by other monocarboxylates. The lactate indicator was not significantly affected by Ca 2+ at the physiological concentrations prevailing in the cytosol, endoplasmic reticulum, and extracellular space, but was affected by Ca 2+ in the low micromolar range. Targeting the indicator to the endoplasmic reticulum revealed for the first time sub-cellular lactate dynamics. Its improved lactate-induced fluorescence response permitted the development of a multiwell plate assay to screen for inhibitors of the monocarboxylate transporters MCTs, a pharmaceutical target for cancer and inflammation. The functionality of the indicator in living tissue was demonstrated in the brain of Drosophila melanogaster larvae. CanlonicSF is well suited to explore lactate dynamics with sub-cellular resolution in intact systems.

RNA binding proteins and messenger RNAs (mRNAs) assemble into ribonucleoprotein granules that regulate mRNA trafficking, local translation, and turnover. The dysregulation of RNA-protein condensation disturbs synaptic plasticity and neuron survival and has been widely associated with human neurological disease. Neuronal granules are thought to condense around particular proteins that dictate the identity and composition of each granule type. Here, we show in Drosophila that a previously uncharacterized long noncoding RNA, mimi , is required to scaffold large neuronal granules in the adult nervous system. Neuronal ELAV-like proteins directly bind mimi and mediate granule assembly, while Staufen maintains condensate integrity. mimi granules contain mRNAs and proteins involved in synaptic processes; granule loss in mimi mutant flies impairs nervous system maturity and neuropeptide-mediated signaling and causes phenotypes of neurodegeneration. Our work reports an architectural RNA for a neuronal granule and provides a handle to interrogate functions of a condensate independently of those of its constituent proteins.