Abstract:
A nanosensor for detecting and quantifying lactate in different types of samples, such as tissues, intra-cellular and subcellular compartments, with high spatial and temporal resolution is disclosed. Methods comprising use of the nanosensor for quantifying the activity of lactate transporters, rates of cellular lactate production and cellular lactate consumption, and rate of mitochondrial pyruvate consumption are also disclosed. Methods for quantifying the transformation in energy metabolism that characterizes cancer cells with single-cell resolution and for detecting interference of candidate drugs with mitochondrial energetics are additionally disclosed.

Description:
[0001]    This application is being filed on 13 Apr. 2012, as a PCT International Patent application in the name of Centro de Estudios Cientificos de Valdivia a Chilean corporation and Carnegie Institution of Washington, a U.S. national corporation, applicant for the designation of all countries except the US, and Luis Felipe Barros Olmedo, Alejandro San Martin, Sebastian Ceballo Charpentier, all citizens of Chile, and Wolf B. Frommer, a citizen of Germany, applicants for the designation of the US only. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention comprises a nanosensor for detecting and quantifying lactate in different types of samples, such as tissues, intra-cellular and even in subcellular compartments, with high spatial and temporal resolution, across four orders of concentration magnitude, and methods that make use of this nanosensor for the quantification of the activity of lactate transporters, for the quantification of the rates of cellular lactate production and cellular lactate consumption, and for the quantification of the rate of mitochondrial pyruvate consumption. Additionally, the invention comprises a method to quantify the transformation in energy metabolism that characterizes cancer cells with single-cell resolution and a method to detect interference of candidate drugs with mitochondrial energetics. 
       BACKGROUND OF THE INVENTION 
       [0003]    Lactate is an organic chemical compound that participates in the metabolism of eukaryotic and prokaryotic cells. Lactate is exchanged between organelles, cells and organs as fuel or waste product, and also plays important signaling and biosynthetic roles, being involved in the physiology of exercise, inflammation, wound healing, neurovascular coupling and also in diseases such as cancer, hypoxic/ischemic disease and microbial infection. In addition, lactate is of industrial interest as a food additive, as a detergent, for the detection and control of microbial growth and for the production of biodegradable polymers. 
         [0004]    Lactate is in dynamic flux between subcellular compartments, between the cell and the extracellular space and between cells. Because the concentration of lactate in the cell compartments is unknown, the dynamics of lactate in the living body is a largely unknown area. 
         [0005]    Standard methods to measure lactate are based on enzymatic reactions, which have to be followed by photometric, amperometric or other devices. Enzyme-based electrodes have been developed that can detect lactate with high-temporal resolution. Another approach to measure lactate is high performance liquid chromatography (HPLC), where lactate is separated from other compounds by passing the sample through a stationary phase stored in a column. There is a problem in the prior art, however, that the existing methods are invasive as they require the extraction of samples or consume lactate, and therefore, they change the concentration of lactate in the sample. A second problem is their sensitivity, since they can not detect the minute amount of lactate present in a single cell or a single subcellular organelle. 
         [0006]    The transport of lactate across cellular and subcellular membranes is mediated by the monocarboxylate transporter (MCT), a molecule involved in the pathogenesis of several diseases and an important target for pharmacological intervention in cancer and diabetes. There are no available methods to measure the transport of lactate in single cells. More specifically, current and common techniques used to measure the transport of lactate using radioactive isotopes cannot resolve single cells and have poor temporal resolution, which hampers the study of fast phenomena and normal tissues, which are heterogeneous in their cellular composition. An existing technique infers the transport of lactate in single cells from changes in pH that accompany the transport of lactate, but this technique is limited insofar as requires prior knowledge of the usually unknown buffering capacity of the cell and is not easily applicable in the presence of physiological bicarbonate buffers. 
         [0007]    The rates of lactate production and lactate consumption are important parameters of cell metabolism, with relevance for hypoxia/ischemia, cancer, diabetes and other pathological conditions. There are no available methods to measure the rates of lactate production and consumption in single cells. More specifically, current and common techniques used to measure the rates of lactate production and consumption are enzyme-based methods that cannot resolve single cells, have poor temporal resolution, and cannot be applied in the presence of physiological concentrations of lactate. Particularly, measurements using isotopes cannot resolve single cells and have poor sensitivity and temporal resolution. Other currently available technique infers the production of lactate by a cell population by following changes in pH that accompany the production of lactate, but this indirect technique is limited insofar as it is affected by other mechanisms affecting extracellular pH and is not easily applicable in the presence of physiological bicarbonate buffers. 
         [0008]    The rate of pyruvate consumption by mitochondria, equivalent under some conditions to the rate of the tricarboxylic acid (TCA) cycle and oxidative phosphorylation, is one of the fundamental parameters of cell metabolism and is affected in several diseases including hypoxic/ischemia, cancer, diabetes and other conditions. There are no available methods to measure the rates of the mitochondrial metabolism in single cells. More specifically, current and common techniques to measure the rates of the mitochondrial metabolism use cannot resolve single cells and have poor sensitivity and low temporal resolution. 
         [0009]    In the state of the art there is no evidence of an optical tool or nanosensor for detecting and quantifying lactate in samples, in tissues and in cellular and subcellular compartments, with high spatial and temporal resolution. Also, there are no available techniques to quantitate single-cell resolution lactate transport or the rates of lactate consumption/production or the rate of mitochondrial metabolism or the Warburg effect, the metabolic transformation that underlies cancer. Nevertheless there are related documents in the art, which will be described below. Sensors for different metabolites are described in W02006096213A1, W02006096214A1, W02006044612A2 and W02007046786A2 that involve a FRET donor, a FRET acceptor and a member of the class of periplasmic binding proteins (PBPs), proteins located in outside bacterial plasma membranes involved in chemotaxis. The periplasmic binding protein serves as the specific recognition element. As there is no known rule to predict whether a given protein may serve as an effective recognition element, these proteins have been the result of informed trial and error, semi-rational design. The current invention does not used any of the recognition elements described W02006096213A1, W02006096214A1, W02006044612A2 or W02007046786A2. Moreover, the current invention is not based on any members of the periplasmic binding protein family but rather on a member of the GntR family, a subclass of transcription factors involved in adaptation of bacteria to changing environmental conditions. Surprisingly, the sensor described in the present invention was found to detect its ligand over 4 orders of magnitude, which makes it unique. PBP-based sensors can only quantify ligands over 2 orders of magnitude only. 
         [0010]    W02001033199A2 discloses a probe based on a target binding site peptide (i) attached to a first fluorescent polypeptide capable of binding to (i) and attached to a second fluorescent polypeptide. The probe includes a linker connecting the two fluorescent polypeptides which allows the distance between them to vary, the fluorescent polypeptides display fluorescence resonance energy transfer (FRET) between them. The probe described in W02001033199A2 is qualitatively different from the probe described in the current invention insofar as the current invention does not involve displacement of binding between two peptides but rather a conformational change elicited by the ligand in a whole protein. 
         [0011]    WO2008008149 describes a method to measure the rates of glycolysis and mitochondrial metabolism in cell populations by recording the rate of extracellular oxygen depletion and the rate of extracellular acidification over minutes using a specific dedicated apparatus. The current invention differs from WO2008008149 as it does not need a dedicated apparatus and can be used with standard multi-well plate readers. It also differs in terms of spatial resolution as it can measure single cells and temporal resolution, which is in the order of seconds. The current invention measures the rate of lactate production directly, whereas WO2008008149 provides an indirect estimate by recording the accumulation of extracellular protons, a parameter that is affected by other processes unrelated to metabolism and that required unphysiological pH buffering conditions. 
         [0012]    WO/2012/002963 describes a method to estimate the rate of glucose consumption in single cells or cell population with high temporal resolution using a FRET glucose nanosensor. The current invention differs from WO/2012/002963 as is does not measure glucose or the rate of glucose consumption but the rates of lactate production/consumption and the rate of mitochondrial metabolism, rates that are independent of the rate of glucose consumption, being a completely different technical application. Moreover, the present method allows an estimation of the Warburg effect, which is not possible with a glucose nanosensor. 
       DISCLOSURE OF THE INVENTION 
       [0013]    The subject of the present invention is to provide a nanosensor, which allows minimally-invasive measurement of lactate over an extended range of lactate concentration with high sensitivity regardless of the concentration of the probe, which does not consume lactate during measurement, and that can be used to measure lactate in samples, in cells and in subcellular compartments. Further, the subject of the present invention is to provide a measuring method of lactate using the nanosensor. Said method can be used to measure the activity of the lactate transporters, to measure the rates of cellular lactate production and lactate consumption, and to measure the rate of pyruvate consumption by mitochondria, which under certain conditions is equivalent to the rate of the tricarboxylic acid (TCA) cycle, a method for single-cell quantification of the Warburg effect, a transformation of metabolism that characterizes cancer cells, and a method to detect interference between drugs and bioenergetic pathways. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0014]    The present invention is related to a genetically-encoded Forster resonance energy transfer (FRET)-based indicator composed of the bacterial L1dR transcription factor sandwiched between any suitable donor and acceptor fluorescent proteins moieties that are capable in combination of serving as donor and acceptor moieties in FRET. Preferred donor and acceptor moieties are selected from the group consisting of mTFP (monomeric teal fluorescent protein), CFP (cyan fluorescent protein), BFP (blue fluorescent protein), GFP (green fluorescent protein), YFP (yellow fluorescent protein), enhanced variations thereof such as enhanced YFP (EYFP), Citrine or Venus, or infrared fluorescent proteins from bacterial phytochromes, with a particularly preferred embodiment provided by the donor/acceptor mTFP/YFP Venus, a variant of YFP with improved pH tolerance and maturation time (Nagai et al., 2002). Criteria to consider when selecting donor and acceptor fluorescent moieties is known in the art, for instance as disclosed in U.S. Pat. No 6,197,928, which is herein incorporated by reference in its entirety. An alternative is the use of a single fluorescent moiety such as circularly-permuted variations of GFP (Akerboom et al., 2008) inserted into the backbone of L1dR or other suitable lactate-binding protein, which undergoes a change in fluorescence intensity in response to binding of lactate to the L1dR moiety or to other suitable lactate-binding protein. In a more preferred embodiment, the fluorescent proteins are mTFP and Venus. 
         [0015]    Unexpectedly, the lactate sensor of the present invention shows a biphasic dose response curve with apparent dissociation constants for lactate of 8 μM and 800 μM, which allows quantitation of lactate over four orders of magnitude (from 10 −6  to 10 −2  M), and differs from all existing FRET metabolite nanosensors, which only allow measurement over two orders of magnitude, for example W02006096213A1, W02006096214A1, W02006044612A2 and W02007046786A2. The invention also comprises methods that exploit the high spatiotemporal resolution of the lactate sensor of the present invention for the measurement of lactate, which, depending on the configuration of the method, allows the measurement of transport activity and of two metabolic rates, the rate of lactate production/consumption and the rate of pyruvate consumption by mitochondria and a method to quantify the Warburg phenomenon in single-cells. These methods can be applied to single cells or cell populations, adherent cells or in suspension, to a cell culture, a tissue culture, a mixed cell culture, a tissue explant, or it can also be applied to animal tissues in vivo. The method comprises the expression of the lactate sensor of the present invention in individual cells. 
         [0016]    The nanosensor of the present invention is expressed in single cells or cell populations, adherent cells or in suspension, in a cell culture, a tissue culture, a mixed cell culture, a tissue explant, or in animal tissues in vivo. The gene expression can be attained by any suitable method to transfer the sensor gene information to the host cell. Examples of gene transfer methodologies are plasmid transfer for instance using liposomal delivery, virus transfer and transgenesis. 
         [0017]    Once the sensor is expressed in single cells or cell populations, adherent cells or in suspension, in a cell culture, a tissue culture, a mixed cell culture, a tissue explant, or in animal tissues in vivo, the sensor is calibrated according to pre-established conditions. In order to express fluorescence data in terms of lactate concentration, a single-point calibration protocol is applied at the end of each experiment. Briefly, intracellular lactate is first lowered by depriving the cells of lactate and glucose, a maneuver that inhibits lactate production at Lactate dehydrogenase (LDH). To ensure that cytosolic lactate is indeed negligible, cells are exposed to pyruvate, which on entering via MCT, increases in the number of inward-facing binding sites available for lactate extrusion, effectively “pumping out” the residual lactate. With the value for the fluorescence ratio at this “zero” lactate condition, the kinetic constants determined in vitro, and the maximum change of fluorescence ratio of 38% or the value determined for a each cell type, fluorescence data are converted into lactate concentration as shown in  FIG. 12 . 
         [0018]    The nanosensor of the invention, is further used in a method for determination of lactate concentrations as described before in single cells or cell populations, adherent cells or in suspension, in a cell culture, a tissue culture, a mixed cell culture, a tissue explant, or in animal tissues in vivo. Depending on the configuration of the method for determination of lactate concentrations, in a first embodiment, the use of the nanosensor of the invention, in a method allows the determination of the lactate transporter activity (i.e. estimation of kinetic parameters of lactate transporter). 
         [0019]    In a second embodiment, the use of the nanosensor of the invention, in a method allows determination of lactate production and/or consumption rates. 
         [0020]    In a third embodiment, the use of the nanosensor of the invention, in a method allows the measurement of mitochondrial pyruvate consumption and/or production rates. 
         [0021]    In a further embodiment, the use of the nanosensor of the invention, in a method for single-cell quantification of the Warburg effect. 
         [0022]    Both the foregoing summary and the following detailed description provide examples and are explanatory only. Accordingly, the foregoing summary and the following detailed description should not be considered to be restrictive. Further, features or variations may be provided in addition to those set forth herein. For example, certain embodiments may be directed to various feature combinations and sub-combinations described in the detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The invention is illustrated by the accompanying drawings wherein: 
           [0024]      FIG. 1  shows the tridimensional structures of the transcriptional regulator L1dR from  E. coli  and  C. glutamicum.    
           [0025]      FIG. 2  shows the amino acid sequences of L1dR from  E. coli  and  C. glutamicum.    
           [0026]      FIG. 3  shows the alignment of the amino acid sequences of sixteen variants of the lactate sensor, SEQ ID NO 1 corresponding to variant 1, SEQ ID NO 2 corresponding to variant 2, SEQ ID NO 3 corresponding to variant 3, SEQ ID NO 4 corresponding to variant 4, SEQ ID NO 5 corresponding to variant 5, SEQ ID NO 6 corresponding to variant 6, SEQ ID NO 7 corresponding to variant 7, SEQ ID NO 8 corresponding to variant 8, SEQ ID NO 9 corresponding to variant 9, SEQ ID NO 10 corresponding to variant 10, SEQ ID NO 11 corresponding to variant 11, SEQ ID NO 12 corresponding to variant 12, SEQ ID NO 13 corresponding to variant 13, SEQ ID NO 14 corresponding to variant 14, SEQ ID NO 15 corresponding to variant 15, SEQ ID NO 16 corresponding to variant 16. 
           [0027]      FIG. 4  shows the response to lactate of sixteen variants of the lactate sensor, wherein the black filled bars correspond to the variants 1, 3, 5, 7, 9, 11, 13, 15, and the grey filled bars correspond to variants 2, 4, 6, 8, 10, 12, 14, and 16. 
           [0028]      FIG. 5  shows the effect of lactate on the fluorescence emission spectrum of the most responsive variant of the sensor, Variant 7, which is encoded by SEQ ID NO 7. 
           [0029]      FIG. 6  presents the change in fluorescence ratio of Variant 7, in response to increasing concentrations of lactate. 
           [0030]      FIG. 7  summarizes the effect of several molecules on the fluorescence ratio of Variant 7, showing the specificity of the nanosensors. 
           [0031]      FIG. 8  shows the effect of pH on the fluorescence ratio of the lactate sensor of the present invention. 
           [0032]      FIG. 9  shows the effect of extracellular lactate on the fluorescence ratio of Variant 7, expressed in HEK293 cells and astrocytes. 
           [0033]      FIG. 10  shows that sensor concentration does not affect the response of Variant 7, to lactate. 
           [0034]      FIG. 11  shows the emission spectra and dose-response of Variant 7, encoded by SEQ ID NO 1, expressed in HEK293 cells. 
           [0035]      FIG. 12  illustrates a one-point calibration protocol for Variant 7, encoded by SEQ ID NO 1. 
           [0036]      FIG. 13  compares the uses of lactate and pH measurements for the characterization of the lactate transporter in astrocytes. 
           [0037]      FIG. 14  depicts the main biochemical pathways for lactate in mammalian cells. 
           [0038]      FIG. 15  demonstrates the measurement of cellular lactate production rate and mitochondrial pyruvate consumption rate in single astrocytes and HEK293 cells. 
           [0039]      FIG. 16  compares metabolic rates measured experimentally with those obtained by fitting a mathematical model to the data. In the equation shown in  FIG. 16 :
       Pyruvate concentration, [Pyr] (μM)   Lactate, [Lac] (μM)   Glycolytic pyruvate production, G (μM/s)   Lactate dexydrogenase forward reaction, LDHf (s-1)   Lactate dehydrogenase reverse reaction, LDHr (s-1)   Cellular lactate release, MCT (μM/s)   Mitochondrial pyruvate uptake, PT (μM/s)       
 
           [0047]      FIG. 17  shows the acute activation of lactate production by inhibition of oxidative phosphorylation. 
           [0048]      FIG. 18  shows the effect of lactate on Variant 7, expressed in T98G glioma cells. 
           [0049]      FIG. 19  plots the lactate production rate and mitochondrial metabolism in individual astrocytes, HEK293 cells and T98G glioma cells. 
           [0050]      FIG. 20  shows the Warburg Index of astrocytes and glioma cells. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0051]    The following detailed description refers to the accompanying drawings. While embodiments of the nanosensor of the invention may be described, modifications, adaptations, and other implementations are possible. For example, substitutions, additions, or modifications may be made to the elements illustrated in the drawings, and the methods described herein may be modified by substituting, reordering, or adding stages to the disclosed methods. Accordingly, the following detailed description does not limit the scope of the invention. While the nanosensor and the methods are described in terms of “comprising” various elements or steps, the nanosensor and the methods can also “consist essentially of” or “consist of” the various elements or steps, unless stated otherwise. Additionally, the terms “a,” “an,” and “the” are intended to include plural alternatives, e.g., at least one, unless stated otherwise. 
         [0052]    The nanosensor quantifies lactate between 1 μM and 10 mM, allowing single-cell measurement of lactate concentration, lactate transporter (MCT) activity, lactate production and the rate of mitochondrial metabolism, as well as detection of the Warburg effect in individual cells. 
         [0053]    The nanosensor of the present invention is a Forster Resonance Energy Transfer (FRET)-based lactate nanosensor further based on L1dR, a bacterial transcription regulator that has two modules, a lactate-binding/regulatory domain and a DNA-binding domain. The L1dR genes were selected from  Corynebacterium glutamicum  and from  Escherichia coli.    
         [0054]    The tridimensional structure of the two proteins is virtually superimposable ( FIG. 1 ), yet they are only 19.4% identical, differing in numerous charged residues ( FIG. 2 ), which may alter surface charge scanning and possibly FRET efficiency. The FRET-based lactate nanosensor of the invention may incorporate any suitable donor and acceptor fluorescent proteins moieties that are capable in combination of serving as donor and acceptor moieties in FRET. Preferred donor and acceptor moieties are selected from the group consisting of mTFP (monomeric teal fluorescent protein), CFP (cyan fluorescent protein), BFP (blue fluorescent protein), GFP (green fluorescent protein), YFP (yellow fluorescent protein), enhanced variations thereof such as enhanced YFP (EYFP), Citrine or Venus, or infrared fluorescent proteins from bacterial phytochromes, with a particularly preferred embodiment provided by the donor/acceptor mTFP/YFP Venus, a variant of YFP with improved pH tolerance and maturation time (Nagai et al., 2002). Criteria to consider when selecting donor and acceptor fluorescent moieties is known in the art, for instance as disclosed in U.S. Pat. No. 6,197,928, which is herein incorporated by reference in its entirety. An alternative is the use of a single fluorescent moiety such as circularly-permuted variations of GFP (Akerboom et al., 2008) inserted into the backbone of L1dR or other suitable lactate-binding protein, which undergoes a change in fluorescence intensity in response to binding of lactate to the L1dR moiety or to other suitable lactate-binding protein. A variant of yellow fluorescent protein with fast and efficient maturation for cell-biological applications. Nagai T, Ibata K, Park E S, Kubota M, Mikoshiba K, Miyawaki A. Nat Biotechnol. 2002 January; 20(1):87-90. Crystal structures of the GCaMP calcium sensor reveal the mechanism of fluorescence signal change and aid rational design. Akerboom J, Rivera J D, Guilbe M M, Malavé E C, Hernandez H H, Tian L, Hires S A, Marvin J S, Looger L L, Schreiter E R. J Biol Chem. 2009 Mar. 6; 284(10):6455-64. Epub 2008 Dec. 18. 
         [0055]    In a more preferred embodiment, the FRET pair selected was mTFP and Venus, which compared with CFP and YFP are respectively brighter and less pH-sensitive. 
         [0056]    The general architecture search for structural combinations of the sensors is shown in  FIG. 1   a , with mTFP located at the N-terminus, the L1dR flanked by linkers, and Venus located at the C-terminus. 
         [0057]    Three constructs were generated for each bacterial species, differing with respect to the presence of DNA binding domain and linkers ( FIG. 3 ). A comparative analysis showed that three proteins that changed their fluorescence in response to lactate, showed that constructs with L1dR from  E. coli  changed their fluorescence ratio much more than those from  C. glutamicum . The list of sequences comprises different embodiments of the invention, which should not be considered as limiting of the invention. 
         [0058]    In a further embodiment, the present invention includes lactate nanosensors described according to the amino acid sequences and have at least 60%, 70%, 80% 85%, 90%, 95%, or 99% sequence identity with SEQ ID NO 1, SEQ ID NO 2, SEQ ID NO 3, SEQ ID NO 4, SEQ ID NO 5, SEQ ID NO 6, SEQ ID NO 7, SEQ ID NO 8, SEQ ID NO 9, SEQ ID NO 10, SEQ ID NO 11, SEQ ID NO 12, SEQ ID NO 13, SEQ ID NO 14, SEQ ID NO 15, or SEQ ID NO 16. 
         [0059]    The present invention also considers the nucleic acid sequences having at least 60%, 70%, 80% 85%, 90%, 95%, or 99% sequence identity with SEQ ID NO 17, SEQ ID NO 18, SEQ ID NO 19, SEQ ID NO 20, SEQ ID NO 21, SEQ ID NO 22, SEQ ID NO 23, SEQ ID NO 24, SEQ ID NO 25, SEQ ID NO 26, SEQ ID NO 27, SEQ ID NO 28, SEQ ID NO 29, SEQ ID NO 30, SEQ ID NO 31, or SEQ ID NO 32. 
         [0060]    The sequences described in SEQ ID NO 1 to SEQ ID NO 16 are only particular embodiments of the present invention provided as way of exemplification of the present invention, and should not be considered to limit the scope of the invention. 
         [0061]    Also surprising was the observation that the DNA-binding domain is important for the FRET change, and that the sensors with no linkers are more responsive ( FIG. 4 ). The most responsive variant, arrowed in  FIG. 4 , was chosen for further characterization. It contains the full length L1dR from  E. coli  and no linkers. The emission spectrum of this nanosensor showed the expected peaks of mTFP and Venus at 492 nm and 526 nm, respectively ( FIG. 5 ). The affinity constant of L1dR for L-lactate is not known.  FIG. 6  shows that this nanosensor responded to a wide range of the ratio between mTFP and Venus. Fluorescence (at 430 nm excitation) was measured at increasing lactate concentrations, behavior well represented by a double rectangular hyperbola, with apparent dissociation constant (KD) values of 8±2 μM and 830±160 μM, and respective maximum ΔR values of 8±0.4% and 11±0.4%. This unique property of L1dR confers the lactate sensor the desirable ability of reporting across four orders of magnitude (from 1 μM to 10 mM), instead of the two orders afforded by one-site sensors. 
         [0062]    When used in vitro, the sensitivity of this nanosensor is similar to at least the most sensitive enzyme-based commercially available kit (50 pmoles). 
         [0063]    The specificity was investigated by exposing the sensor to millimolar levels of several organic acids and glucose, of which only lactate induced a significant change in FRET ( FIG. 7 ). The sensor showed a modest sensitivity to pH in the physiological range ( FIG. 8 ). Expressed in mammalian cells, the lactate sensor of the present invention distributed in the cytosol and was excluded from nuclei and organelles ( FIG. 9 ). Compared to the glucose sensor, its distribution was more heterogeneous, possibly due to L1dR multimerization, but this did not affect the response to lactate (Supplementary  FIG. 10 ). Expressed in cells, the sensor showed emission spectra and two-component dose-response curve similar to that observed in vitro, but with a larger change in FRET ratio ( FIG. 11 ). In order to express fluorescence data in terms of lactate concentration, a single-point calibration protocol is applied at the end of each experiment. Briefly, intracellular lactate is first lowered by depriving the cells of lactate and glucose, a maneuver that decreases the glycolytic flux and lowers the cytosolic NADH:NAD+ ratio (Hung et al., 2011; Zhao et al., 2011), inhibiting lactate production at Lactate dehydrogenase (LDH). To ensure that cytosolic lactate is indeed negligible, we use a property of MCTs termed trans-acceleration or accelerated exchange (Halestrap and Price, 1999). Cells are exposed to pyruvate, which on entering via MCT, increases in the number of inward-facing binding sites available for lactate extrusion, effectively “pumping out” the residual lactate. With the value for the fluorescence ratio at this “zero” lactate condition, the kinetic constants determined in vitro, and the maximum change of fluorescence ratio of 38% or the value determined in the specific cell type, fluorescence data were converted into lactate concentration as shown in  FIG. 12 . After 20 minutes of glucose/lactate deprivation in HEK293 cells or neurons, or 1 hour deprivation in astrocytes, intracellular lactate is undetectable (data not shown), consistent with the very low NADH:NAD+ ratio present under such conditions (Hung et al., 2011; Zhao et al., 2011). 
         [0064]    The invention further comprises methods using the aforementioned nanosensor for determination of lactate concentrations in single cells or cell populations, adherent cells or in suspension, in a cell culture, a tissue culture, a mixed cell culture, a tissue explant, or in animal tissues in vivo. 
         [0065]    The method comprises the general steps of:
       a) Expressing the nanosensor of the invention, in a desired host, such as single cells or cell populations, adherent cells or in suspension, in a cell culture, a tissue culture, a mixed cell culture, a tissue explant, or in animal tissues in vivo;   b) Calibrating the host with predetermined values of intracellular, extracellular, subcellular lactate concentrations, recording lactate concentrations in time;   c) Disrupting the steady-state of lactate entering the cell;   d) Recording the output from the nanosensor calculating the lactate concentration at different time points;       
 
         [0070]    In the step b), corresponding to calibrating the host, the nanosensor of the invention is calibrated in cells using the kinetic constants of the sensor obtained in vitro and a zero-lactate level determined in the presence of pyruvate. Pyruvate can be in the range of 5 mM to 20 mM, preferentially 10 mM. 
         [0071]    The general method can be applied in different configurations, for example, in a first embodiment, the nanosensor is used in a method for the measurement of the activity of the lactate transporter. 
         [0072]    In this first embodiment, with the information obtained in the calibration step, the disruption of the steady-state of lactate entering the cell is carried out by altering the extracellular concentration of lactate, thus exposing the cells to lactate. This causes a rise in intracellular lactate that is monitored with the lactate sensor and whose initial rate is independent of lactate metabolism and can be used to estimate kinetic parameters. Exposure of the cells to increasing concentrations of lactate allows the estimation of kinetic parameters for the lactate transporter. Kinetic parameters are also obtained from the decrease in intracellular lactate after removal of extracellular lactate. 
         [0073]    In a second embodiment, the general method can be applied to a method to measure the rates of lactate production and lactate consumption. In this second embodiment, with the information obtained in the calibration step, the steady-state of lactate is disrupted by altering the function of lactate transporter, for example by addition of a blocker of the lactate transporter. In mammalian cells, the lactate transporter is the MCT and can be blocked with phloretin, parachloromercurybenzoate or other suitable compounds. If the cell is a net lactate producer, application of the MCT-blocker causes an increase in intracellular lactate concentration, the initial rate of which is equal to the rate of cellular lactate production in the steady-state. If the cell is a net lactate importer, application of the MCT-blocker causes a fall in intracellular lactate concentration, the initial rate of which is equal to the rate of lactate consumption on the steady-state. In a more particular embodiment, the disruption of the steady-state is attained by adding an inhibitor of the MCT, such as, but not limited to phloretin, parachloromercurybenzoate, anti-MCT antisera, etc. In cells where lactate transport is mediated by other transporters, the method can be applied using their respective inhibitors. A critical property of this nanosensor that allows quantitation of these rates is its high temporal resolution, for only the initial rate of lactate accumulation is informative and after a few seconds other non-linear processes like inhibition of glycolysis by the increasing lactate or changes in mitochondrial pyruvate uptake may interfere with the measurement. Because of its low temporal resolution, extracellular lactate measurement by existing techniques cannot be used in combination with MCT-blockage to estimate the rates of lactate production or lactate consumption. 
         [0074]    In a third embodiment, the general method can be applied to a method to measure the rate of mitochondrial pyruvate consumption. 
         [0075]    In this third embodiment, with the information obtained in the calibration step, the disruption in the lactate steady-state is caused by disrupting the flux of lactate. To quantitate the rate of mitochondrial pyruvate consumption, the steady-state is disrupted by addition of a blocker of the mitochondrial pyruvate transporter. In mammalian cells, the mitochondrial pyruvate transporter can be blocked with low concentration of 4-CIN. In cells, the concentration of pyruvate and lactate move together as a single pool because of fast interconversion by the high activity enzyme lactate dehydrogenase (LDH), with lactate representing over 90% of the pool. Application of the pyruvate transporter-blocker 4-CIN or other suitable inhibitor of the mitochondrial pyruvate transporter, causes an increase in the intracellular lactate concentration, the initial rate of which is equal to the rate of pyruvate uptake in the steady-state. In cells were pyruvate uptake into mitochondria were mediated by other transporters, the method could be applied using their respective inhibitors. A critical property of this nanosensor that allows quantitation of these rates is its high temporal resolution, for only the initial rate of lactate accumulation is informative and after a few seconds other non-linear processes MCT-transport and inhibition of glycolysis by increasing lactate may interfere with the measurement. In the steady-state and in the presence of glucose and lactate as exclusive oxidative substrates, the rate of pyruvate consumption by mitochondria is equal to the rate of the tricarboxylic acid (TCA) cycle and equal to rate of oxidative phosphorylation (OXPHOS). 
         [0076]    A fourth particular embodiment of the method of the present invention is determination of cancer staging by estimation of the ratio between lactate production and the rate of the TCA cycle in a sample. Cancer cells are less oxidative than normal cells, a phenomenon known as the Warburg effect, which is receiving renewed attention regarding cancer pathogenesis, diagnosis and possibly treatment. Robust flux through glycolysis and pentose phosphate pathways in these cells are thought to provide building blocks for proliferation and a high redox tone, while the lactic acid exported acidifies the environment and facilitates tumor migration and metastasis. A plot of lactate production versus TCA cycle rate shows that T98G glioma cells can be distinguished from normal astrocytes ( FIG. 3   a - b ), but a more sensitive parameter is the ratio between lactate production and the rate of the TCA cycle, which we have termed Warburg Index ( FIG. 3   c ). In an alternative embodiment, the Warburg Index is estimated by calculating the ratio between lactate production (with phloretin or other MCT blocker) and the rate of intracellular lactate increase in response to inhibition of oxidative phosphorylation with azide or other suitable compound ( FIG. 17 ), which is a parameter of how oxidative is the cell. This alternative version of the Warburg index gives a different value but is also very sensitive to the mitochondrial defects that characterize cancer cells, senescent cells and other conditions that produce the Warburg phenomenon. 
         [0077]    These tools allow the functional study of cancer metabolism with single-cell resolution and are also readily adaptable to multi-well format for high-throughput analysis of metabolism in cancer and other diseases. The development of a sensor based on L1dR provides the basis for creating a wide variety of novel indicators because the GntR superfamily, of which L1dR is a member, has 270 other transcription factors that bind pyruvate, fatty acids, amino acids, TCA cycle intermediates, etc., which are possible candidates to serve as templates for genetically-encoded nanosensors. 
         [0078]    Based on the lactate nanosensor of this invention, methods are presented that allow for the first time single-cell real-time quantification of the rates of lactate production and of the tricarboxylic acid (TCA) cycle. Both methods follow cytosolic accumulation of lactate immediately after blockage of selected transporters, in analogous fashion to the measurement of the rate of glucose consumption with a glucose sensor. In the steady-state, the intracellular concentration of lactate is kept constant by a dynamic balance between glycolytic production and lactate efflux ( FIG. 2   c ). Perturbation of the steady-state by addition of an MCT blocker like phloretin is expected to cause intracellular lactate accumulation at a rate equal to the rate of lactate production. For net lactate importers, like liver cells and possibly neurons, the MCT blocker should decrease intracellular lactate at a rate equal to the rate of lactate consumption. A similar rationale can be applied to the quantification of pyruvate consumption by mitochondria. The high activity of lactate dehydrogenase (LDH) in mammalian cells, couples the concentrations of lactate and pyruvate, which for this purposes can be considered as a single pool, with lactate representing &gt;90%. Acute inhibition of the mitochondrial pyruvate transporter (PT) with a low concentration of α-Cyano-4-hydroxycinnamate (4-CIN) should produce an accumulation of intracellular lactate, at the rate of pyruvate consumption, which in the absence of alternative mitochondrial substrates is equivalent to the rates of the TCA cycle and oxidative phosphorylation. Experimental demonstration of these methods is provided in  FIG. 2   d . On average, astrocytes presented a lactate production rate of 2 μ/s and a TCA cycle rate of 7.6 μM/s, consistent with their rate of glucose consumption of 2-6 μM/s 11 . Typical of cell lines, HEK293 cells were more glycolytic and less oxidative than astrocytes, with respective rates of lactate production and TCA cycle of 5.4 and 2.1 μM/s. Inhibition of mitochondrial ATP production with sodium azide caused a 26±4 fold increase in the rate of lactate production, fitting the deviation of all pyruvate flux towards lactate production and the 3-4 fold increase observed in glucose consumption ( FIG. 2   e ). Fitting a mathematical model to actual lactate measurements indicated that the initial slopes of the changes in lactate concentration triggered by the transport blockers underestimate the actual rates of lactate production and TCA cycle by less than 10% (Supplementary  FIG. 7 ). 
         [0079]    The following examples are provided to help in the understanding of the present invention, and should not be considered a limitation to the scope of the invention. 
       EXAMPLES 
       [0080]    In order to help understanding the invention, the present invention will be explained with reference to specific examples: 
         [0081]    Protein Purification. Plasmid constructs were transformed into  E. coli  BL21 (DE3). A single colony was inoculated in 100 ml of LB medium with 100 mg/ml ampicillin (without IPTG) and shaken in the dark for 2-3 days. Cells were collected by centrifugation at 5000 rpm (4° C.) for 10 min and disrupted by sonication (Hielscher Ultrasound Technology) in 5 mL of Tris-HCl buffer pH 8.0. A cell-free extract was obtained by centrifugation at 10,000 rpm (4° C.) for 1 hour and filtering of the supernatant (0.45 μm). Proteins were purified using a Nickel resin (His Bin® from Novagen) as recommended by the manufacturer. Eluted proteins were quantified using the Biuret method and stored at −20° C. in 20% glycerol. The variant that showed the largest change in fluorescence ratio, was cloned into pcDNA3.1(−) for expression in eukaryotic cells using the restriction sites BamHI and HindIII. 
         [0082]    Animals and Cell Cultures. Animals used were mixed F1 male mice (C57BL/6J×CBA/J), kept in an animal room under Specific Pathogen Free (SPF) conditions at a room temperature of 20±2° C., in a 12/12 h light/dark cycle with free access to food and water. Experiments were approved by the Centro de Estudios Cientificos Animal Care and Use Committee. Mixed cortical cultures of neuronal and glial cells (1-3 day-old neonates) were prepared as described (Loaiza et al., 2003). HEK293 and T98G glioma cells were acquired from the American Tissue Culture Collection and cultured at 37° C. in 95% air/5% CO 2  in DMEM/F12 10% fetal bovine serum. Cultures were transfected at 60% confluence using Lipofectamine 2000 (Gibco) or alternatively, exposed to 5×10 6  PFU of Ad lactate sensor of the present invention (Vector Biolab), and studied after 24-72 h. 
         [0083]    Fluorescence Measurements. Nickel-purified proteins were resuspended at 100 nM in an intracellular buffer containing (mM): 10 NaCl, 130 KCl, 1.25 MgSO4 and 10 HEPES, pH 7.0, and measured with a microplate reader analyzer (EnVision, PerkinElmer). The proteins were excited at 430 nm and the intensity of fluorescence emission of mTFP and Venus were recorded at 485 nm (FmTFP) and 528 nm (FVenus), respectively. The ratio (R) between FmTFP and FVenus was used to characterize the sensors. Emission spectra were obtained at 430 nm excitation, with 2 nm windows. Cells were imaged at room temperature (22-25° C.) in a 95% air/5% CO 2 -gassed solution of the following composition (in mM): 112 NaCl, 1.25 CaCl 2 , 1.25 MgSO 4 , 1-2 glucose, 10 HEPES, 24 NaHCO 3 , pH 7.4, with 3 mM KCl (astrocytes) or 5 mM KCl (HEK and T98G) using an upright Olympus FV1000 Confocal Microscope equipped with a 20× water immersion objective (N.A. 1.0) and a 440 nm solid-state laser. Alternatively, cells were imaged with an Olympus IX70 or with an Olympus BX51 microscope equipped with a 40× oil-immersion objective (NA 1.3) or with a 20× water-immersion objective (NA 0.95). Microscopes were equipped with CAIRN monochromators (Faversham, UK), and either a Hamamatsu Orca camera controlled by Kinetics software or a Rollera camera controlled with Metafluor software, respectively. For nanosensor ratio measurements, cells were excited at 430 nm for 0.2-0.8 s. Emission was divided with a CAIRN Optosplit, equipped with band pass filters at 480±20 (FmTFP) and 535±15 nm (FVenus). The ratio between FmTFP and FVenus was used to measure lactate. The pH-sensitive dye BCECF was ester loaded at 0.1 μM for 3-4 min and the signal was calibrated by exposing the cultures to solutions of different pH after permeabilizing the cells with 10 μg/ml nigericin and 20 μg/ml gramicidin in an intracellular buffer. BCECF was sequentially excited at 440 and 490 nm (0.05 s) and imaged at 535±15 nm. 
         [0084]    Mathematical Modeling of Lactate Dynamics. A model of intracellular lactate dynamics was generated according to the flux diagram in  FIGS. 14 and 16 , in the absence of extracellular lactate, 
         [0000]        d [Pyr]/ dt =( G +[Lac]*LDHr−[Pyr]*LDHf−PT)/vol  d [Lac]/ dt =([Pyr]*LDHf−[Lac]*LDHr−VMCT/{KMCT+[Lac]})/vol
 
         [0000]    where [Pyr] is cytosolic pyruvate concentration, [Lac] is cytosolic lactate concentration, G is glycolytic pyruvate production, LDHf and LDHr are the lactate dehydrogenase forward and reverse reactions and PT is mitochondrial pyruvate uptake. MCT efflux obeys Michaelis-Menten kinetics with maximum rate VMCT and an apparent affinity KMCT (5 mM). The kinetic model was solved numerically with the computer software Berkeley Madonna using the Rosenbrock method. 
         [0085]    Statistical Analysis. All time courses correspond to single cells. Experiments were repeated three to six times, with 6-12 cells per experiment. Regression analyses were carried out with the computer program SigmaPlot (Jandel). Differences in mean values of paired samples were evaluated with the Student&#39;s t-test. P values &lt;0.05 were considered significant and are indicated with an asterisk (*). 
         [0086]    Sixteen different variants of the lactate nanosensor, according to different embodiments of the present invention were produced.  FIG. 4  shows the response to lactate of the sixteen variants of the lactate sensor, wherein the black filled bars correspond to the variants 1, 3, 5, 7, 9, 11, 13, 15, and the grey filled bars correspond to variants 2, 4, 6, 8, 10, 12, 14, and 16. Each of the produced variants of the lactate nanosensor of the present invention are encoded by the aminoacid sequence described in the list, SEQ ID NO 1 corresponding to variant 1, SEQ ID NO 2 corresponding to variant 2, SEQ ID NO 3 corresponding to variant 3, SEQ ID NO 4 corresponding to variant 4, SEQ ID NO 5 corresponding to variant 5, SEQ ID NO 6 corresponding to variant 6, SEQ ID NO 7 corresponding to variant 7, SEQ ID NO 8 corresponding to variant 8, SEQ ID NO 9 corresponding to variant 9, SEQ ID NO 10 corresponding to variant 10, SEQ ID NO 11 corresponding to variant 11, SEQ ID NO 12 corresponding to variant 12, SEQ ID NO 13 corresponding to variant 13, SEQ ID NO 14 corresponding to variant 14, SEQ ID NO 15 corresponding to variant 15, SEQ ID NO 16 corresponding to variant 16. Most of the variants showed a measureable change in fluorescence ratio in response to lactate and may be used for the different methods described in the present invention. The high rate of successful sensor generation shows a surprising robustness of L1dR as a scaffold for FRET-based sensor generation. 
       Example 1 
     Method for the Measurement of Lactate Transporter Activity with High Spatiotemporal Resolution 
       [0087]    By controlling the exchange of lactate between cells and the interstitial space, MCTs are nodal points of tissue metabolism. MCTs catalyze the stoichiometric translocation of lactate and a proton and their activity can be measured with single-cell resolution by monitoring intracellular pH with a dye such as BCECF. However, 99.9% of protons are bound to proteins, phospholipids and other sites, and are exchanged through many transporters other than the MCT, which makes pH an imperfect proxy for lactate. To compare the performances of the lactate sensor of the present invention and BCECF, we chose astrocytes. When expressed in astrocytes, the lactate sensor of the present invention responded well to extracellular lactate, allowing real-time monitoring of lactate influx and efflux ( FIG. 13 ). Consistent with an MCT-mediated process, the initial rate of astrocytic uptake of 1 mM lactate of 1.6±0.5 μM/s was inhibited by 96±1% in the presence of the MCT blocker phloretin (50 μM). In contrast, exposure to extracellular lactate produced only a small change in intracellular pH as detected with BCECF ( FIG. 15 ). Thus, the lactate sensor can be used to measure MCT, allowing a more sensitive and physiological characterization of their function. Lactate may also be transported independently of protons through gap junctions (Rouach et al., 2008) and possibly through connexin hemichannels and pannexin channels, fluxes that are invisible to pH measurements and that may now be measured with the present invention. 
       Example 2 
     Metabolic Rate of Pyruvate Consumption by Mitochondria 
       [0088]    The diagram in  FIG. 14  illustrates how the intracellular concentration of lactate is determined by the dynamic balance between pyruvate production by glycolysis, pyruvate consumption by mitochondria and lactate exchange through MCTs. In cells that are exporting lactate, perturbation of the steady state by addition of a blocker of the MCT is expected to cause lactate accumulation. In cells that are net lactate importers, an MCT-blocker is expected to cause depletion in intracellular lactate. In both cases, the rate of change will be equal to the rate of lactate production or consumption. As a demonstration of the principle in HEK293 cells and in astrocytes, MCT inhibition with phloretin (50 μM) caused the expected increase in intracellular lactate, indicative of lactate production ( FIG. 15 ). Phloretin is also known to inhibit GLUT glucose transporters, however this should not compromise the analysis of astrocytes, neurons, or the cell lines so far characterized, which maintain resting intracellular glucose at levels well above the Km of hexokinase (Bittner et al., 2010; Takanaga et al., 2008; Fehr et al., 2003). In these cells, glucose consumption remains constant for several minutes in the presence of glucose transporter blockers like phloretin or cytochalasin B (Bittner et al., 2010). Thus, during the first few minutes of phloretin application, the rate of lactate accumulation is not diminished by lack of glucose supply. In muscle cells and adipocytes, which maintain low levels of intracellular glucose, a more selective MCT inhibitor may be used (Ovens et al., 2010). 
       Example 3 
     Method to Measure the Rate of Mitochondrial Metabolism with High Spatiotemporal Resolution 
       [0089]    Because the reaction catalyzed by LDH is relatively fast, the cytosolic pools of lactate and pyruvate are tightly linked, and variations in pyruvate are faithfully mimicked by lactate. Accordingly, perturbation of the steady-state by addition of a blocker of the mitochondrial pyruvate transporter (PT) will cause intracellular lactate accumulation at a rate equal to the rate of pyruvate consumption by mitochondria. As predicted by the kinetic model, inhibition of the mitochondrial pyruvate transporter in HEK293 cells with α-cyano-4-hydroxycinnamate (4-CIN) at a concentration that does not affect MCT function (Halestrap and Denton, 1975), led to an increase in intracellular lactate ( FIG. 15 ). As typical of cell lines, HEK293 cells were more glycolytic than oxidative, having respective rates of lactate production and pyruvate uptake of 5.4 and 2.1 μM/s, whereas on average, astrocytes demonstrated a lactate production of 2 μM/s and pyruvate uptake of 7.6 μM/s ( FIG. 15 ), consistent with their rate of glucose consumption of 2-6 μM/s (Bittner et al., 2010; Bittner et al., 2011). 
         [0090]    To further validate methods 2 and 3, the blockers were applied sequentially and a mathematical model based on the kinetic model described in  FIG. 14  was fitted to the data. The responses of intracellular lactate to transient inhibitions of the PT with 4-CIN (200 μM) and the MCTs with phloretin (50 μM) were measured in the same HEK293 cell in the presence of 25 mM glucose and no extracellular lactate. The straight lines represent the slopes of the lactate increases fitted by linear regression during the first minute (4-CIN) or during the whole exposure (phloretin). The red line represents the best fit of the kinetic model to the data, as described in Experimental Procedures, assuming full inhibition of the transporters. Fitted parameters were: G=10.3 μM/s, PT=4.8 μM/s, LDHf=9 s-1, LDHr=0.45 s-1, Vmax=186 μM/s, G-PT (lactate production)=5.5 μM/s. PT and lactate release rates respectively estimated from the initial slopes of lactate increase after transporter inhibition were over 90% of those estimated by modeling. As shown in  FIG. 16 , the rate of lactate accumulation induced by 4-CIN was maximal at the onset of inhibition and then declined, due to increased efflux through the MCT as lactate accumulated. Therefore, only the initial rate of lactate accumulation will represent mitochondrial metabolism accurately. In contrast, MCT blockage resulted in sustained accumulation of lactate, a finding that is consistent with the accepted notions that glycolytic pyruvate production and mitochondrial pyruvate consumption are not modulated by cytosolic pyruvate. On the other hand, the limited accumulation of lactate caused by 4-CIN confirms that 4-CIN did not block MCTs to a significant extent. The best fit of the model to the data showed that the initial slopes of the changes in lactate concentration triggered by the transport blockers underestimate the actual rates of mitochondrial pyruvate uptake and lactate production by less than 10%. 
       Example 4 
     Detection of Drugs that Interfere with Mitochondrial Metabolism with High Spatiotemporal Resolution 
       [0091]    The screening for unwanted effects is an important part of the process of drug discovery. One possibility to be ruled out before the drug is tested in animals or humans is the possibility that a candidate drug may exert undesirable effects on cellular energy metabolism. An inhibition of mitochondrial ATP production is compensated by increase in glycolytic ATP production and lactate production. Typically, a 3-4 fold increase in the rate of glucose consumption is observed (Bittner et al., 2010). However, the increase in lactate production can be much higher, because without the mitochondrial pyruvate sink, all glucose is now converted into lactate. Taking advantage of the improved resolution of the lactate sensor of the present invention, a method is presented that detects mitochondrial poisoning with very high sensitivity. As an example of this method, an acute inhibition of oxidative phosphorylation in astrocytes with 5 mM azide caused a 26±4-fold increase in the rate of lactate production measured with the lactate sensor of the present invention ( FIG. 17   a ).  FIG. 17   b  shows the acute effect of azide 5 mM on the intracellular concentration of lactate. Used in multi-well plate format, both protocols may be incorporated in high throughput applications for the screening of mitochondrial interference. 
       Example 5 
     Detection of the Warburg Effect in Single Cells with High Temporal Resolution 
       [0092]    Augmented flux through glycolysis and the pentose phosphate pathway in cancer cells provides the building blocks for proliferation and a high redox state that protects them against free radicals released during chemotherapy, while the lactic acid exported via MCTs acidifies the tumor environment and facilitates cell migration and metastasis. The glycolytic nature of cancer cells even in the presence of oxygen, a phenomenon known as the Warburg effect, is detected by comparing lactate production with oxygen consumption, measurements that demand large numbers of cells and overlook tissue heterogeneity. The reversible nature of mitochondrial flux and lactate production measurements with the lactate sensor allowed a more refined characterization of the Warburg phenotype. A comparison of astrocytes with T98G glioblastoma cells, showed that the non-transformed cells are more oxidative than their tumor counterparts ( FIGS. 18 and 19 ). The difference between normal and cancerous cells was dramatically amplified by lactate production and pyruvate uptake, to give a parameter of cell metabolism that we have termed Warburg Index (WI). Some glioblastoma cells behaved almost like an astrocyte but some presented Warburg Index values that were 100 times higher than that of a normal astrocyte ( FIG. 20 ). Tumors are known to be metabolically heterogeneous, which is expected given unequal access of their cells to oxygen and nutrients, but it seems remarkable that a cell line like T98G, cultured under carefully controlled conditions at high oxygen levels be also so heterogeneous from the metabolic point of view. The single-cell real-time capability of the lactate sensor should allow a metabolic characterization of individual cells and cell lineages in tumors and tissue explants. Used in cell populations with a multi-well plate reader, it is readily amenable for high throughput applications. An alternative embodiment of Example 5 replaces 4-CIN with an inhibitor of oxidative phosphorylation like azide or rotenone. 
         [0093]    While certain embodiments of the invention have been described, other embodiments may exist. Further, any disclosed method steps or stages may be modified in any manner, including by reordering steps and/or inserting or deleting steps, without departing from the invention. While the specification includes a detailed description of the nanosensor and the associated drawings, the invention&#39;s scope is indicated by the following claims. Furthermore, while the specification has been described in a specific language, the claims are not limited to the features or acts described above. Rather, the specific features and acts described above are disclosed as illustrative aspects and embodiments of the invention. Various other aspects, embodiments, modifications, and equivalents thereof which, after reading the description herein, may suggest themselves to one of ordinary skill in the art without departing from the spirit of the present invention or the scope of the claimed subject matter. 
       LIST OF REFERENCES 
       [0000]    
       
         Brooks, G. A. J. Physiol 587, 5591-5600 (2009). 
         Barros, L. F. &amp; Deitmer, J. W. Brain Res. Rev. 63, 149-159 (2010). 
         Ganapathy, V., Thangaraju, M., &amp; Prasad, P. D. Pharmacol. Ther. 121, 29-40 (2009). 
         Vander Heiden, M. G., Cantley, L. C., &amp; Thompson, C. B. Science. 324, 1029-1033 (2009). 
         Pellerin, L. et al. Glia. 55, 1251-1262 (2007). 
         Gao, Y. G. et al. Nucleic Acids Res. 36, 7110-7123 (2008). 
         Aguilera, L. et al. J. Bacteriol. 190, 2997-3005 (2008). 
         Deuschle, K. et al. Protein Sci. 14, 2304-2314 (2005). 
         Day, R. N., Booker, C. F., &amp; Periasamy, A. J. Biomed. Opt. 13, 031203 (2008). 
         Nagai, T. et al. Nat. Biotechnol. 20, 87-90 (2002). 
         Bittner, C. X. et al. Front. Neuroenergetics 2, 1-11 doi: 10.3389/fnene.2010.00026 (2010). 
         Ovens, M. J. et al. Biochem. J. 425, 523-530 (2010). 
         Becker, H. M., Broer, S., &amp; Deitmer, J. W. Biophys. J. 86, 235-247 (2004). 
         Halestrap, A. P. &amp; Denton, R. M. Biochem. J. 148, 97-106 (1975). 
         Georgi, T., Engels, V., &amp; Wendisch, V. F. J. Bacteriol. 190, 963-971 (2008). 
         Nagai T, Ibata K, Park E S, Kubota M, Mikoshiba K, Miyawaki A. Nat Biotechnol. 2002 January; 20(1):87-90. 
         Akerboom J, Rivera J D, Guilbe M M, Malave E C, Hernandez H H, Tian L, Hires S A, Marvin J S, Looger L L, Schreiter E R. J Biol Chem. 2009 Mar. 6; 284(10):6455-64. Epub 2008 Dec. 18.