Channelrhodopsin-2-XXL, a powerful optogenetic tool for low-light applications.

Channelrhodopsin-2 (ChR2) has provided a breakthrough for the optogenetic control of neuronal activity. In adult Drosophila melanogaster, however, its applications are severely constrained. This limitation in a powerful model system has curtailed unfolding the full potential of ChR2 for behavioral neuroscience. Here, we describe the D156C mutant, termed ChR2-XXL (extra high expression and long open state), which displays increased expression, improved subcellular localization, elevated retinal affinity, an extended open-state lifetime, and photocurrent amplitudes greatly exceeding those of all heretofore published ChR variants. As a result, neuronal activity could be efficiently evoked with ambient light and even without retinal supplementation. We validated the benefits of the variant in intact flies by eliciting simple and complex behaviors. We demonstrate efficient and prolonged photostimulation of monosynaptic transmission at the neuromuscular junction and reliable activation of a gustatory reflex pathway. Innate male courtship was triggered in male and female flies, and olfactory memories were written through light-induced associative training.

Degradation of channelopsin-2 in the absence of retinal and degradation resistance in certain mutants.

Channelrhodopsin-2 is a light-gated cation channel from the green alga Chlamydomonas reinhardtii. It is functional in animal cells and therefore widely used for light-activated depolarization, especially in neurons. To achieve a fully functional protein, the chromophore all-trans-retinal is needed. It has not been investigated whether or not the apoprotein is stable without its cofactor until now. Here we show that channelopsin-2 (Chop2, protein without bound retinal) is much more prone to degradation than channelrhodopsin-2 (protein with retinal). Constructs of Chop2 fused to yellow fluorescent protein (Chop2::YFP) in the absence and presence of retinal confirm this observation by exhibiting strongly differing fluorescence. We present mutants of Chop2 with highly increased stability in the absence of retinal. Substitution of threonine 159 with aromatic amino acids causes enhanced resistance to degradation in the absence of retinal, which is confirmed by fluorescence intensity, the increase in photocurrents on the addition of retinal to previously expressed protein, and Western blot analysis. Exchanging threonine 159 with cysteine, however, increases photocurrents due to better binding of retinal, without obvious stabilization against degradation of the retinal-free opsin. We also show that the light-activated hyperpolarizing chloride pump halorhodopsin from Natronomonas pharaonis (NpHR) is not prone to retinal-dependent degradation.

CCM3 interacts with CCM2 indicating common pathogenesis for cerebral cavernous malformations.

Individuals carrying a mutation in one of the three cerebral cavernous malformation genes (CCM1/KRIT1, CCM2, CCM3) cannot be clinically distinguished, raising the possibility that they act within common molecular pathways. In this study, we demonstrate that CCM3 (PDCD10) coprecipitates and colocalizes with CCM2. We also show that CCM3 directly binds to serine/threonine kinase 25 (STK25, YSK1, SOK1) and the phosphatase domain of Fas-associated phosphatase-1 (FAP-1, PTPN13, PTP-Bas, PTP-BL). CCM3 is phosphorylated by STK25 but not by its other Yeast-Two hybrid interactor STK24, whereas the C-terminal catalytic domain of FAP-1 dephosphorylates CCM3. Finally, our experiments reveal that STK25 forms a protein complex with CCM2. Thus, our data link two proteins of unknown function, CCM3 and STK25, with CCM2, which is part of signaling pathways essential for vascular development and CCM pathogenesis.