Engineering Genetically Encoded Nanosensors for Real-Time In Vivo Measurements of Citrate Concentrations

Citrate is an intermediate in catabolic as well as biosynthetic pathways and is an important regulatory molecule in the control of glycolysis and lipid metabolism. Mass spectrometric and NMR based metabolomics allow measuring citrate concentrations, but only with limited spatial and temporal resolution. Methods are so far lacking to monitor citrate levels in real-time in-vivo. Here, we present a series of genetically encoded citrate sensors based on Förster resonance energy transfer (FRET). We screened databases for citrate-binding proteins and tested three candidates in vitro. The citrate binding domain of the Klebsiella pneumoniae histidine sensor kinase CitA, inserted between the FRET pair Venus/CFP, yielded a sensor highly specific for citrate. We optimized the peptide linkers to achieve maximal FRET change upon citrate binding. By modifying residues in the citrate binding pocket, we were able to construct seven sensors with different affinities spanning a concentration range of three orders of magnitude without losing specificity. In a first in vivo application we show that E. coli maintains the capacity to take up glucose or acetate within seconds even after long-term starvation

Combining Membrane Potential Imaging with l-Glutamate or GABA Photorelease

Combining membrane potential imaging using voltage sensitive dyes with photolysis of l-glutamate or GABA allows the monitoring of electrical activity elicited by the neurotransmitter at different sub-cellular sites. Here we describe a simple system and some basic experimental protocols to achieve these measurements. We show how to apply the neurotransmitter and how to vary the dimension of the area of photolysis. We assess the localisation of photolysis and of the recorded membrane potential changes by depolarising the dendrites of cerebellar Purkinje neurons with l-glutamate photorelease using different experimental protocols. We further show in the apical dendrites of CA1 hippocampal pyramidal neurons how l-glutamate photorelease can be used to calibrate fluorescence changes from voltage sensitive dyes in terms of membrane potential changes (in mV) and how GABA photorelease can be used to investigate the phenomenon of shunting inhibition. We also show how GABA photorelease can be used to measure chloride-mediated changes of membrane potential under physiological conditions originating from different regions of a neuron, providing important information on the local intracellular chloride concentrations. The method and the proof of principle reported here open the gateway to a variety of important applications where the advantages of this approach are necessary

High-resolution simultaneous voltage and Ca2+ imaging

Combining voltage and Ca2+ imaging allows the correlation of electrical and chemical activity at sub-cellular level. Here we describe a novel apparatus designed to obtain simultaneous voltage and Ca2+ measurements with single-trial resolution from sites as small as a few microns. These measurements can be obtained with negligible optical cross-talk between the two signals and negligible photo-damage of the preparation. The capability of the technique was assessed recording either from individual neurons in brain slices or from networks of cultured neurons. The present achievements open the gate to many novel physiological investigations requiring simultaneous measurement of voltage and Ca2+ signals

Human Renal Cell Carcinoma Induces a Dendritic Cell Subset That Uses T-Cell Crosstalk for Tumor-Permissive Milieu Alterations

Tissue dendritic cells (DCs) may influence the progression of renal cell carcinoma (RCC) by regulating the functional capacity of antitumor effector cells. DCs and their interaction with T cells were analyzed in human RCC and control kidney tissues. The frequency of CD209+ DCs in RCCs was found to be associated with an unfavorable TH1 cell balance in the tissue and advanced tumor stages. The CD209+ DCs in RCC were unusual because most of them co-expressed macrophage markers (CD14, CD163). The phenotype of these enriched-in-renal-carcinoma DCs (ercDCs) could be reiterated in vitro by carcinoma-secreted factors (CXCL8/IL-8, IL-6, and vascular endothelial growth factor). ErcDCs resembled conventional DCs in costimulatory molecule expression and antigen cross-presentation. They did not suppress cognate cytotoxic T-lymphocyte function and did not cause CD3ζ down-regulation, FOXP3 induction, or T-cell apoptosis in situ or in vitro; thus, they are different from classic myeloid-derived suppressor cells. ErcDCs secreted high levels of metalloproteinase 9 and used T-cell crosstalk to increase tumor-promoting tumor necrosis factor α and reduce chemokines relevant for TH1-polarized lymphocyte recruitment. This modulation of the tumor environment exerted by ercDCs suggests an immunologic mechanism by which tumor control can fail without involving cytotoxic T-lymphocyte inhibition. Pharmacologic targeting of the deviated DC differentiation could improve the efficacy of immunotherapy against RCC