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  • G01N33/53—Immunoassay; Biospecific binding assay; Materials therefor
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  • G01N33/542—Immunoassay; Biospecific binding assay; Materials therefor with immune complex formed in liquid phase with steric inhibition or signal modification, e.g. fluorescent quenching
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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  • G01N33/502—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing involving human or animal cells for testing or evaluating the effect of chemical or biological compounds, e.g. drugs, cosmetics for testing non-proliferative effects
  • G01N33/5035—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing involving human or animal cells for testing or evaluating the effect of chemical or biological compounds, e.g. drugs, cosmetics for testing non-proliferative effects on sub-cellular localization
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  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/48—Biological material, e.g. blood, urine; Haemocytometers
  • G01N33/50—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing
  • G01N33/5005—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing involving human or animal cells
  • G01N33/5008—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing involving human or animal cells for testing or evaluating the effect of chemical or biological compounds, e.g. drugs, cosmetics
  • G01N33/502—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing involving human or animal cells for testing or evaluating the effect of chemical or biological compounds, e.g. drugs, cosmetics for testing non-proliferative effects
  • G01N33/5038—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing involving human or animal cells for testing or evaluating the effect of chemical or biological compounds, e.g. drugs, cosmetics for testing non-proliferative effects involving detection of metabolites per se
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/48—Biological material, e.g. blood, urine; Haemocytometers
  • G01N33/50—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing
  • G01N33/58—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing involving labelled substances
  • G01N33/582—Chemical analysis of biological material, e.g. blood, urine; Testing involving biospecific ligand binding methods; Immunological testing involving labelled substances with fluorescent label

Definitions

• 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.
• 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.
• 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.
• 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.
• HPLC high performance liquid chromatography
• 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.
• MCT monocarboxylate transporter
• 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.
• the rate of pyruvate consumption by mitochondria is one of the fundamental parameters of cell metabolism and is affected in several diseases including hypoxic/ischemia, cancer, diabetes and other conditions.
• TCA tricarboxylic acid
• WO2006096214A1, WO2006044612A2 and WO2007046786A2 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.
• PBPs periplasmic binding proteins
• 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 WO2006096213A1, WO2006096214A1,
• 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.
• 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.
• WO2001033199A2 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.
• FRET fluorescence resonance energy transfer
• the probe described in WO2001033199A2 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.
• 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.
• 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.
• the present method allows an estimation of the Warburg effect, which is not possible with a glucose nanosensor.
• 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.
• TCA tricarboxylic acid
• the present invention is related to a genetically-encoded Forster resonance energy transfer (FRET)-based indicator composed of the bacterial LldR 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.
• FRET Forster resonance energy transfer
• 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.
• the lactate sensor of the present invention shows a biphasic dose response curve with apparent dissociation constants for lactate of 8 ⁇ and 800 ⁇ , which allows quantitation of lactate over four orders of magnitude (from 10 "6 to 10 " M), and differs from all existing FRET metabolite nanosensors, which only allow measurement over two orders of magnitude, for example WO2006096213A1, WO2006096214A1, WO2006044612A2 and WO2007046786A2.
• 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.
• 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.
• 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.
• 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).
• LDH Lactate dehydrogenase
• the nanosensor of the invention is further used in a method for
• 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.
• 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).
• the use of the nanosensor of the invention in a method allows determination of lactate production and/or consumption rates.
• the use of the nanosensor of the invention in a method allows the measurement of mitochondrial pyruvate consumption and/or production rates.
• the use of the nanosensor of the invention in a method for single-cell quantification of the Warburg effect.
• Figure 1 shows the tridimensional structures of the transcriptional regulator LldR from E. coli and C. glutamicum.
• Figure 2 shows the amino acid sequences of LldR from E. coli and C.
• Figure 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.
• Figure 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.
• Figure 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.
• Figure 6 presents the change in fluorescence ratio of Variant 7, , in response to increasing concentrations of lactate.
• Figure 7 summarizes the effect of several molecules on the fluorescence ratio of Variant 7, showing the specificity of the nanosensors.
• Figure 8 shows the effect of pH on the fluorescence ratio of the lactate sensor of the present invention.
• Figure 9 shows the effect of extracellular lactate on the fluorescence ratio of Variant 7, expressed in HEK293 cells and astrocytes.
• Figure 10 shows that sensor concentration does not affect the response of Variant 7, to lactate.
• Figure 11 shows the emission spectra and dose-response of Variant 7, encoded by SEQ ID NO 1, expressed in HEK293 cells.
• Figure 12 illustrates a one-point calibration protocol for Variant 7, encoded by SEQ ID NO 1.
• Figure 13 compares the uses of lactate and pH measurements for the characterization of the lactate transporter in astrocytes.
• Figure 14 depicts the main biochemical pathways for lactate in mammalian cells.
• Figure 15 demonstrates the measurement of cellular lactate production rate and mitochondrial pyruvate consumption rate in single astrocytes and HEK293 cells.
• Figure 16 compares metabolic rates measured experimentally with those obtained by fitting a mathematical model to the data. In the equation shown in Figure 16:
• Figure 17 shows the acute activation of lactate production by inhibition of oxidative phosphorylation.
• Figure 18 shows the effect of lactate on Variant 7, expressed in T98G glioma cells.
• Figure 19 plots the lactate production rate and mitochondrial metabolism in individual astrocytes, HEK293 cells and T98G glioma cells.
• Figure 20 shows the Warburg Index of astrocytes and glioma cells.
• the nanosensor quantifies lactate between 1 ⁇ 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.
• MCT lactate transporter
• the nanosensor of the present invention is a Forster Resonance Energy Transfer (FRET)-based lactate nanosensor further based on LldR, a bacterial transcription regulator that has two modules, a lactate-binding/regulatory domain and a DNA-binding domain.
• FRET Forster Resonance Energy Transfer
• the LldR genes were selected from Corynebacterium glutamicum and from Escherichia coli.
• 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.
• Crystal structures of the GCaMP calcium sensor reveal the mechanism of fluorescence signal change and aid rational design.
• the FRET pair selected was mTFP and Venus, which compared with CFP and YFP are respectively brighter and less pH- sensitive.
• Fig. 1 a The general architecture search for structural combinations of the sensors is shown in Fig. 1 a, with mTFP located at the N-terminus, the LldR flanked by linkers, and Venus located at the C-terminus.
• 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.
• 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.
• 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.
• the sensitivity of this nanosensor is similar to at least the most sensitive enzyme-based commercially available kit (50 pmoles).
• 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.
• 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;
• 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 20mM, preferentially 10 mM.
• the nanosensor is used in a method for the measurement of the activity of the lactate transporter.
• 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.
• the general method can be applied to a method to measure the rates of lactate production and lactate consumption.
• 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.
• the lactate transporter is the MCT and can be blocked with phloretin,
• the disruption of the steady-state is attained by adding an inhibitor of the MCT, such as, but not limited to phloretin,
• parachloromercurybenzoate 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.
• the general method can be applied to a method to measure the rate of mitochondrial pyruvate consumption.
• the disruption in the lactate steady-state is caused by disrupting the flux of lactate.
• the steady- state is disrupted by addition of a blocker of the mitochondrial pyruvate transporter.
• the mitochondrial pyruvate transporter can be blocked with low concentration of 4-CIN.
• 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.
• LDH lactate dehydrogenase
• 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.
• 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.
• the rate of pyruvate consumption by mitochondria is equal to the rate of the
• TCA tricarboxylic acid
• OXPHOS oxidative phosphorylation
• 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. 3a-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. 3c).
• Fig. 3c Warburg Index
• 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.
• 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
• LDH dehydrogenase
• PT mitochondrial pyruvate transporter
• 4-CIN a-Cyano-4-hydroxycinnamate
• astrocytes presented a lactate production rate of 2 ⁇ s and a TCA cycle rate of 7.6 ⁇ /s, consistent with their rate of glucose consumption of 2 - 6 ⁇ /s 11 .
• 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 ⁇ /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. 2e).
• 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).
• 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
• Lipofectamine 2000 (Gibco) or alternatively, exposed to 5 x 10 6 PFU of Ad lactate sensor of the present invention (Vector Biolab), and studied after 24-72 h.
• Nickel-purified proteins were resuspended at 100 nM in an intracellular buffer containing (raM): 10 NaCl, 130 KC1, 1.25 MgS04 and 10 HEPES, pH 7.0, and measured with a microplate reader analyzer (En Vision, 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% C0 2 -gassed solution of the following composition (in mM): 112 NaCl, 1.25 CaCl 2 , 1.25 MgS0 4 , 1-2 glucose, 10 HEPES, 24 NaHCO 3 , pH 7.4, with 3 mM KC1 (astrocytes) or 5 mM KC1 (HEK and T98G) using an upright Olympus FV1000 Confocal Microscope equipped with a 20x water immersion objective (N.A. 1.0) and a 440 nm solid-state laser.
• N.A. 1.0 20x water immersion objective
• 440 nm solid-state laser 440 nm solid-state laser.
• cells were imaged with an Olympus 1X70 or with an Olympus BX51 microscope equipped with a 40x oil-immersion objective (NA 1.3) or with a 20x 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.
• CAIRN monochromators Faversham, UK
• a Hamamatsu Orca camera controlled by Kinetics software or a Rollera camera controlled with Metafluor software respectively.
• 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 ⁇ for 3-4 min and the signal was calibrated by exposing the cultures to solutions of different pH after permeabilizing the cells with 10 ⁇ nigericin and 20 ⁇ gramicidin in an intracellular buffer. BCECF was sequentially excited at 440 and 490 nm (0.05 s) and imaged at 535 ⁇ 15 nm.
• d[Lac]/dt ([Pyr]*LDHf- [Lac]*LDHr -VMCT/ ⁇ KMCT+[Lac] ⁇ )/vol
• [Pyr] is cytosolic pyruvate concentration
• [Lac] is cytosolic lactate concentration
• G glycolytic pyruvate production
• LDHf and LDHr are the lactate dehydrogenase forward and reverse reactions
• PT mitochondrial pyruvate uptake.
• MCT efflux obeys Michaelis-Menten kinetics with maximum rate VMCT and an apparent affinity KMCT (5 raM). The kinetic model was solved numerically with the computer software Berkeley Madonna using the Rosenbrock method.
• Figure 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
• Example 1 Method for the measurement of lactate transporter activity with high spatiotemporal resolution.
• MCTs are nodal points of tissue metabolism. MCTs catalyze the
• lactate and a proton and their activity can be measured with single-cell resolution by monitoring intracellular pH with a dye such as BCECF.
• a dye such as BCECF.
• 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.
• 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).
• the initial rate of astrocytic uptake of 1 mM lactate of 1.6 ⁇ 0.5 ⁇ /s was inhibited by 96 ⁇ 1 % in the presence of the MCT blocker phloretin (50 ⁇ ).
• exposure to extracellular lactate produced only a small change in intracellular pH as detected with BCECF (Fig. 15).
• 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.
• 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.
• perturbation of the steady state by addition of a blocker of the MCT is expected to cause lactate accumulation.
• an MCT-blocker is expected to cause depletion in intracellular lactate.
• the rate of change will be equal to the rate of lactate production or consumption.
• MCT inhibition with phloretin (50 ⁇ ) 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.
• Example 4 Detection of drugs that interfere with mitochondrial metabolism with high spatiotemporal resolution.
• 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.
• Example 5 Detection of the Warburg effect in single cells with high temporal resolution.
• 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.
• 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.

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