Abstract:
The present invention relates to a genetically encoded FRET-based biosensor to monitor the activity of matrix metalloproteinase 9 (MMP-9). MMP-9 is an extracellular acting endopeptidase implicated in both physiological and pathological processes. A genetically encoded FRET biosensor anchored in the cellular membrane allows studying the proteolytic activity of MMP-9 with high spatiotemporal resolution at the exact region of MMP-9 action on the cell. Applicability of the biosensor, both in vitro and in vivo in living cells, has been demonstrated by ratiometric analysis of cleavage of the biosensor by a purified auto-activating mutant of MMP-9.

Description:
[0001]    The present invention relates to genetically encoded FRET-based matrix metalloproteinase 9 (MMP-9) activity biosensor and use thereof. 
       PRIOR ART 
       [0002]    Advances in the elucidation of mechanisms governing basic cellular functions have allowed to shift the researchers&#39; focus towards cellular dynamics and created a growing demand for methods that would be sensitive and sufficiently quick to track dynamic processes within living cells. In the area of subcellular spatiotemporal localization of macromolecules interactions, the Förster Resonance Energy Transfer (FRET) based approaches are particularly useful. 
         [0003]    Recently a significant number and diversity of genetically encoded FRET based biosensors that have been developed. They can be used to study such diverse phenomena as ion concentration [Miranda et al., 2012; Burdette et al., 2001; Esposito et al. 2008], organic compound concentration [Gruenwald et al., 2012], GTPase activity [Kalab and Soderholm, 2010], protein phosphorylation [Violin et al., 2003] and mechanical stress within the cell [Meng, F. and F. Sachs, 2011]. Their advantage is the ability to track these phenomena in real time in living cells and organisms. 
         [0004]    MMP-9 is an extracellularly secreted 92 kDa protease that belongs to a family of zinc- and calcium-dependent endopeptidases. It cleaves a number of extracellular matrix proteins and cell adhesion molecules. Large body of research indicates that MMP-9 plays a significant role in the development of cancer through its dual role of regulating angiogenesis and cleaving the extracellular matrix, thus enabling tumors to enter metastasis [Klein and Bischoff, 2011; Deryugina and Quigley, 2006; Kessenbrock et al., 2010]. An increase in MMP-9 expression was observed in a number of different tumors in comparison to healthy subjects with an apparent positive link between the tumor aggressiveness and the MMP-9 activity levels [Hanemaaijer et al., 2000; Schmalfeldt et al., 2001]. MMP-9 processing of the extracellular matrix may also lead to the release of cytokines and growth factors [Schonbeck, et al., 1998; Yu and Stamenkovic, 2000] that facilitate angiogenesis. In fact research points to MMP-9 activity levels being a possible prognostic factor in cancer (see Klein, et al., 2004, for a review). 
         [0005]    Current research also points to MMP-9 as one of the key regulators in synaptic plasticity and, by extension, in processes that are believed to be dependent on synaptic plasticity—learning and memory. Synaptic plasticity is the ability to alter the connection strength between synapses within the brain. 
         [0006]    Commonly employed methods of detecting the MMP-9 proteolytic activity such as DQ-gelatin (a gelatin derivative heavily tagged with fluorescein with its fluorescence almost completely quenched when intact—gelatinase activity results in an increase in fluorescence) or gel/in situ zymography do not allow to assess the localization of gelatinase activity with a high spatiotemporal resolution. Moreover, they are both gelatin-based and detect the activity of MMP-2, another member of the gelatinase subfamily of matrix metalloproteinases (along with MMP-9). Since the MMP-2 expression levels are much higher than those of MMP-9, the majority of cleavage detected by these methods comes from the proteolytic activity of MMP-2 rather than MMP-9. The other recently created fluorescent MMP-9 activity biosensors are completely synthetic [Fudala, et al., 2011; Akers, et al., 2012; Roopali Roy, et al., 2011; Hawkins, et al., 2013; Leight et al., 2013] and as such were intended to be used in a manner akin to DQ-gelatin. 
         [0007]    DQ-protein (dye-quenched) substrates are widely used MMP-9 activity probes that are the available commercially. The DQ substrates are analogs of natural substrates that have been excessively tagged with fluorescent dyes. The close proximity of dye molecules caused by this excessive tagging is responsible for the quenching of the fluorescent signal of an intact substrate. Hydrolysis of the DQ substrate by MMP-9 leads to separation of the dye molecules and an increase in the fluorescence signal. DQ-gelatin and DQ-collagen IV have been successfully used to detect protease activity in vitro (gelatinase activity assay tests), on gel and in situ zymography and in live cell imaging microscopy as well (see Cavallo-Medved, et al., 2009 and Sameni, et al., 2009). The DQ-gelatin and DQ-collagen IV enabled the visualization of ECM degradation and intracellular tracking of degradation products in living cells. That, however, was the limit of their usefulness as the identification of proteases responsible for the degradation and cellular compartments that contained both these proteases and degradation products required additional techniques. DQ substrates offer at best only a global measure of proteolytic activity, even if used in conjunction with live cell imaging. Cleaved by multiple proteases, of little use when local, ultrastructural morphological changes are concerned and unsuitable for in vivo imaging, DQ-protein substrates will not facilitate future studies into fundamental physiological and pathological roles of MMP-9. 
         [0008]    Therefore a number of new molecular probes have been designed that strive to tackle the insufficiencies of DQ-protein substrates. Because of the potential of MMPs as a prognostic markers of cancer, considerable work has been done in the area of diagnostic and analytical probes for the detection of the proteolytic activity of MMP-9 (see Roopali Roy, et al., 2011, for a survey of several MMP-9 activity probes and their suitability as a diagnostic tool to detect cancer and Scherer, et al., 2008 for a general review of the recent developments in the area of MMP activity detection in cancer). Near-infrared or NIRF probes have generated considerable interest thanks to the body permeability to the near-infrared light and thus a potential to be used in vivo, though positron emission tomography probes have also been developed. Many of the NIRF probes display self-quenching properties to increase the signal to noise ratio. A cyclic peptide NIRF probe called Cy5.5-C6 developed in 2009 [Wang, et al., 2009] and originally intended to detect MMP-2 activity was successfully used to correlate the progression of colorectal cancer with the levels of MMP-2 and MMP-9 [Lee, et al., 2012]. The unique property of that probe is that it binds to MMP-9 and inhibits its activity. The commercially available MMPSense 680 is another NIRF probe that was used to detect MMP-9 in vivo [Kaijzel, et al., 2010, and Wallis de Vries, et al., 2009], though the probe itself is fairly indiscriminate when it comes to protease specificity—it is cleaved by MMP-2, -3, -9 and -13. Finally by injecting mice carrying xenograft tumors with a self-assembling triple-helical near-infrared probe discussed in Akers, et al., 2012, researchers observed a robust (five-fold) increase in tumor-associated fluorescence 24 hour post-injection. The fluorescence was diminished with the administration of GM6001 broad-spectrum metalloproteinase inhibitor. 
         [0009]    While the NIRF probes described above are a clear advance over DQ-protein substrates, they were designed with a much narrower scope in mind—as a diagnostic tool rather than a research one. Coupled with a low protease specificity displayed by some of them, undesirable MMP inhibitory properties of others and dependence on synthetic fluorochromes, it is obvious that they are not suitable for the localization of MMP-9 activity on the level of cellular ultrastructure (plasma membrane domains, dendritic spines, etc.). 
         [0010]    The MMP-9 activity probe described in Fudala, et al., 2011 and Fudala, et al., 2012, solves the problem of low protease specificity by utilizing an artificial MMP-9 cleavage site. This analytical probe consists of two fluorescent dyes (5-FAM and Cy5) separated by a short peptide cleavable by MMP-9. In an intact probe the FRET causes the fluorescence of 5-FAM to be quenched and Cy5 fluorescence to be strongly enhanced. MMP-9 cleavage (but not MMP-2) results in the separation of fluorochromes and a significant increase in the fluorescence of 5-FAM. Though the MMP-9 activity probe developed by Fudala et al. is more than likely to be used in a clever way in the coming years (as was the case with DQ-protein substrates) it suffers from the same shortcoming as NIRF probes, which preclude it from being utilized to study fundamental roles of MMP-9 in physiological and pathological conditions. 
         [0011]    Understanding various functions performed by MMP-9 in physiological (extracellular matrix remodeling, angiogenesis, synaptic plasticity) as well as pathological conditions (malignant progression of tumors, epilepsy) requires tools that would enable the assessment of its proteolytic activity. Recent years have witnessed the development of a number of functional imaging techniques that provide the means to measure and localize the MMP-9 activity in living cells. However, all of these methods depend on an exogenously applied fluorescent probe that can be cleaved by MMP-9. A genetically encoded, membrane-anchored, FRET-based MMP-9 activity biosensor might be better suited to elucidate the role of proteolytic activity of MMP-9 in physiological and pathological processes. Therefore, there exist a need to develop a biosensor that would enable investigation of the proteolytic activity of MMP-9 with high spatiotemporal resolution at the exact region of MMP-9 action on the cell that could be used both in vitro and in vivo in living cells. 
       INVENTION SUMMARY 
       [0012]    The present invention provides a genetically encoded FRET-based MMP-9 activity biosensor that overcomes the limitations of the currently available biosensors and makes investigation of fundamental physiological and pathological roles of MMP-9 possible. 
         [0013]    A genetically encoded MMP-9 activity biosensor of the invention comprises a FRET donor fluorescent protein and two FRET acceptor fluorescent proteins separated by flexible linkers, wherein the linker between the donor and acceptor proteins contains a synthetic MMP-9 cleavage site and the entire biosensor is anchored in the plasma membrane. 
         [0014]    Preferably the FRET donor fluorescent protein in the biosensor of the invention is selected from a group comprising a monomeric teal fluorescent protein mTFP1 [Li and Elledge, 2007] and Clover fluorescent protein [Lam et al., 2012]. Most preferably the FRET donor fluorescent protein is mTFP1. mTFP1 is a protein generated from a tetrameric cyan fluorescent protein cFP484 isolated from the coral belonging to the genus  Clavularia.    
         [0015]    Preferably the FRET acceptor fluorescent protein is selected from a group comprising Venus Fluorescent Protein and mRuby2 fluorescent protein. Most preferably the FRET acceptor fluorescent protein is the Venus Fluorescent Protein. Venus Fluorescent Protein is an improved variant of the Yellow Fluorescent Protein. 
         [0016]    The donor-acceptor FRET pair to be used in the biosensor of the invention should be characterized by high Förster radius and photostability. 
         [0017]    The MMP-9 cleavage site in the biosensor of the invention preferably is a synthetic cleavage site. Examples of amino acid sequences that can be used as the MMP-9 cleavage site, i.e. these sequences that are capable of being cleaved by MMP-9, include PRSLS [Fudala, et al., 2011], PLGLAG [Tsien 2013], PLFYSV, KIPRTLT, PLRLSW and PRAVST, KGPRQIT [Kridel, et al., 2001]. The preferred MMP-9 cleavage site comprises PRSLS sequence. 
         [0018]    In the preferred embodiment the biosensor of the invention is anchored in the plasma membrane by a transmembrane domain of the Platelet-derived growth factor receptor (PDGFR). 
         [0019]    Preferably the flexible linkers in the biosensor of the invention are selected from a group consisting of an α-helical linker, a linker comprising one to eight repeats of GGTGGT hexapeptides, a linker comprising one to eight repeats of GGSGSR hexapeptides. 
         [0020]    In one preferred embodiment, the linker between the donor protein and acceptor protein is the α-helical linker, which preferably comprises sequence EEEIREAFRVFPRSLSLRHVMTNL (the site of α-helices is indicated in bold). 
         [0021]    Alternatively the linker between the donor protein and acceptor protein, in addition to the MMP-9 cleavage site, comprises only one GGTGGT or GGSGSR hexapeptide and preferably it comprises only one GGTGGT hexapeptide. This linker is also referred to as a loop-like linker. In the most preferred embodiment, the linker consists of the following sequence: LKGSPRSLSKLK(GGTGGT/GGSGSR)LK (the MMP-9 cleavage site is indicated in bold). 
         [0022]    The other linker, i.e. the linker between two acceptor proteins, comprises preferably seven repeats of GGSGSR hexapeptide. 
         [0023]    The structures of the particularly preferred biosensors of the invention have been schematically presented in  FIG. 1A . In one embodiment the biosensor of the invention comprises two Venus Fluorescent Proteins separated by a flexible linker comprising seven repeats of GGSGSR hexapeptide, wherein one of the Venus Fluorescent Proteins is separated from mTFP1 by an α-helical linker comprising a synthetic MMP-9 cleavage site. In other preferred embodiment the biosensor of the invention comprises two Venus Fluorescent Proteins separated by a flexible linker comprising seven repeats of GGSGSR hexapeptide, wherein one of the Venus Fluorescent Proteins is separated from mTFP1 by linker comprising a synthetic MMP-9 cleavage site and only one GGTGGT hexapeptide. 
         [0024]    The biosensors of the invention exhibit high FRET efficiency and selectivity for MMP-9. The MMP-9 cleavage of the biosensor leads to the release of the acceptor proteins from the cell membrane and a decrease in FRET observed as a drop in the acceptor to donor fluorescence intensity ratio. Therefore, the invention also relates to the use of the genetically encoded MMP-9 activity biosensor of the invention as a system for investigation of the proteolytic activity of MMP-9 in vitro and in vivo in living cells. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0025]      FIG. 1A  presents the general structures of the preferred biosensors of the invention; in one embodiment (labeled (i)) Venus Fluorescent Proteins are separated by 7 GGSGSR repeats and Venus FP and mTFP1 by an α-helical linker comprising an MMP-9 cleavage site; in another embodiment (labeled (ii)) Venus Fluorescent Proteins are separated by 7 GGSGSR repeats and Venus FP and mTFP1 by MMP-9 cleavage site and 1 GGTGGT hexapeptide; variant sensors with lower number of hexapeptide repeats between fluorescent proteins were generated through partial cleavage of the genetic sequence of the sensor.  FIG. 1B  presents FRET efficiency values calculated from AP; light gray bars indicate sensors analyzed with FLIM.  FIG. 1C  presents live cell imaging of HEK293 expressing the biosensor of the invention.  FIG. 1D  presents fractionation results of HEK293 cell line expressing the biosensor of the invention; WB against biosensor was carried out using anti-cmyc antibody; control WBs are presented below. 
           [0026]      FIG. 2A  presents images of HEK293 cells expressing the biosensor with a loop-like linker before and after AP; mTFP1 was excited with the 458 nm laser, Venus with 514 nm laser; right side of the panel presents emission spectra of the optimized biosensor before and after AP; the disappearance of the acceptor peak and the increased fluorescence intensity of the donor are visible.  FIG. 2B  presents AP results of the biosensor having an α-helical linker. 
           [0027]      FIG. 3  presents in vitro cleavage of the sensor with an α-helical linker and negative controls—baseline cleavage, cleavage with the inactive MMP-9 and effects of GM6001 metalloproteinase inhibitor on the biosensor cleavage with the autoactivating MMP-9. Biosensor cleavage was analysed on WB with the anti-GFP antibody; on quantification plots intensity was normalized to full length biosensor in control lanes; intensity of the cleavage product in the control lane was subtracted from cleavage product intensities in the 30 min., 1 h, 4 h, o/n lanes prior to normalization. 
           [0028]      FIG. 4  presents spectra recorded for HEK293 cells expressing biosensor with an α-helical linker before and after the treatment with autoactivating MMP-9 (aaMMP-9); the treatment causes a spectrum shift towards shorter wavelengths indicating cleavage of the sensor. Error bars represent SEM values. Insert presents a maximum projection of a representative lambda stack of a HEK293 cell expressing the biosensor. A rectangle indicates an approximate region of interest, from which the emission spectrum was acquired. For each cell 3 ROIs were used to calculate the average emission spectrum. 
           [0029]      FIG. 5  presents images taken during live imaging of a HEK293 cells expressing the biosensor with α-helical linker. Linear unmixing was performed on cell images acquired on the Zeiss LSM780 to remove an influence of a spectral overlap between mTFP1 and Venus FP. The unmixed data was then used to calculate fluorescence intensity ratios for each pixel of the image in each timepoint of the experiment.  FIG. 5A  presents a map correlating the fluorescence intensity ratio values (OY axis) and fluorescence intensity in that pixel (OX axis). The map was generated for the cell presented in  FIG. 5B .  FIG. 5B  presents images of a HEK293 cell taken at the indicated timepoints. Each pixel was assigned color and hue corresponding to the fluorescence intensity ratio of that pixel. Autoactivating MMP-9 was added to the culture at 5 min. A gradual decrease in the Venus/mTFP1 fluorescence intensity ratio was observed.  FIG. 5C  presents fluorescence intensity ratios of cells either mock treated, treated with inactive MMP-9 or autoactivating MMP-9. The values were normalized to an average fluorescence intensity ratio value calculated for timepoint before the start of the treatment. Each line represents an averaged ratio of 3 cells. The gradual decrease in the ratio values of cells treated with auto-activating MMP-9 was observed. No such decrease is observed in cells that were either mock treated or treated with inactive MMP-9. Error bars represent SEM values. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    The genetically encoded FRET-based MMP-9 activity biosensor of the invention fills an important niche in the field of MMP-9 detection. Until now the MMP-9 activity probes were created with clinical diagnosis in mind, which is understandable, given the role of MMP-9 in the development of cancer. However, as recent years have shown, MMP-9 plays a critical role in other processes, for example in the physiology of the brain. Although probes, such as DQ-gelatin, have been incredibly useful in studying that aspect of MMP-9 role, they quite simply do not offer the required spatiotemporal resolution needed to answer numerous questions that have arisen. A genetically encoded, membrane-anchored FRET-based MMP-9 activity biosensor of the invention is better suited to elucidate the role of proteolytic activity of MMP-9 in physiological and pathological processes. 
         [0031]    The biosensor of the invention utilizes a novel monomeric teal fluorescent protein (mTFP1) that possesses superior spectral properties to CFP [Day, et al., 2008]. It has previously been shown that mTFP1 forms a more efficient FRET pair with the Yellow Fluorescent Protein [Li and Elledge, 2007]. However, other fluorescent proteins, such as Clover FP [Lam et al., 2012], can also be used in the biosensor of the invention. 
         [0032]    mTFP1 serves as a donor of energy while dual Venus Fluorescent Proteins serve as energy acceptors. Venus Fluorescent Protein, an improved variant of the Yellow Fluorescent Protein, was selected for the FRET acceptor. Other FP can also be used as acceptor FP, for example mRuby2 disclosed by Lam et al., supra. 
         [0033]    Although the majority of FRET-based sensors contain one donor protein and one acceptor protein, two acceptor FP proteins were introduced into the sensor to increase its FRET efficiency. FRET efficiency of a single donor and single acceptor system is defined as: 
         [0000]    
       
         
           
             
               
                 E 
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                     k 
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                     k 
                     F 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where k T  is the energy transfer rate, k R  is the rate constant of all other deactivation processes and k F  is the fluorescence decay rate. 
         [0034]    FRET efficiency of a single donor and two acceptors system is given by the following equation: 
         [0000]    
       
         
           
             
               E 
               2 
             
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                   2 
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                     k 
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                     k 
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               . 
             
           
         
       
     
         [0000]    Introduction of a second acceptor in a FRET biosensor increases its FRET efficiency, since 
         [0000]    
       
         
           
             
               E 
               2 
             
             = 
             
               
                 
                   2 
                    
                   
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                     1 
                   
                 
               
               ≥ 
               
                 
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                 . 
               
             
           
         
       
     
         [0035]    The distance between fluorescent proteins in the biosensor of the invention was optimized to maximize the effective FRET efficiency. Since FRET efficiency is determined not only by the distance between the donor and acceptor proteins, but also by the orientation, flexible linkers formed from several GGSGGS or GGTGGT repeats were employed and proved to be efficient in improving the FRET efficiency. Two glycine residues give the linker its flexibility, while a larger amino acid determines linear distance between proteins. 
         [0036]    The other factor determining the functionality of the biosensor of the invention is the MMP-9 cleavage sequence. A screen of known MMP-9 substrates has yielded neither a consensus sequence nor a secondary structure assumed by the MMP-9 cleavage site. Kridel et al. [Kridel, et al., 2001] reported a family of short peptides cleavable by MMP-9 using the phage display technology in an ELISA format. Thus the Inventors have selected a consensus sequence N′-PRSLS-C′ suggested therein, which has been cloned into the biosensor of the invention. This sequence has previously been shown [Fudala et al., 2011] as being indeed recognized by MMP-9. However, other amino acid sequences can also be used as MMP-9 cleavage site (as it has been discussed above). 
         [0037]    As the proteolytic cleavage efficiency strongly depends on the accessibility of the cleavage site, which in turn is influenced by its secondary and tertiary structures, the Inventors have decided to alter the structure of the linker positioned between mTFP1 and Venus Fluorescent Protein in the biosensor of the invention, by placing the MMP-9 cleavage site between two α-helices. 
         [0038]    The Inventors have carried out the acceptor photobleaching (AP) experiments in order to rapidly screen generated biosensors and give a general indication of whether FRET occurs. These experiments were never intended to provide a reliable quantification of the FRET phenomenon. In-depth analysis of the FRET properties of biosensors with the highest FRET efficiency was performed with fluorescence lifetime imaging microscopy (FLIM). There is a slight discrepancy between FRET efficiency values calculated from AP and FLIM experiments. However, the FRET efficiency values based on AP are calculated for the entire cell (therefore they include the cytoplasm, where FRET is negligible due to the membranous localization of the biosensor), whereas FRET efficiency values based on FLIM measurements were calculated for much smaller regions, where FRET was most pronounced. 
         [0039]    As the result of this analysis, it was found that the biosensor with α-helical linker has a higher FRET efficiency than the biosensor with a loop-like linker. This difference is likely due to different higher order structures assumed by loop-like and α-helical linkers, with the latter one being more compact and thus bringing the donor and acceptors closer to each other. 
         [0040]    The MMP-9 proteolytic activity leads to the release of dual Venus proteins from the cell membrane and a decrease in the Venus to mTFP1 fluorescence ratio. The biosensor is present predominantly in the cell membrane. The presence of a small fraction of the biosensor in the cytoplasm may indicate some level of sensor degradation. The biosensor is cleaved by MMP-9 in an in vitro assay. The cleavage is not due to a spontaneous degradation of the protein and can be blocked by the addition of a broad spectrum MMP inhibitor. A baseline cleavage of the biosensor is observable in untreated lysate. Since cell lysis was performed without protease inhibitors of any kind, the observed cleavage may be caused by endogenous MMP-9 present in the HEK293 cell lysate. The baseline cleavage can be, at least partially, blocked by matrix metalloproteinase inhibitors—both broad-spectrum (GM6001 that blocks MMP-1, MMP-2, MMP-3, MMP-8 and MMP-9) and specific (Inhibitor I that selectively blocks MMP-9 and MMP-13). 
         [0041]    The fluorescence emission spectra collected from cell membranes of fixed HEK293 cells expressing the biosensor and treated with the auto-activating MMP-9 differ from those recorded from untreated cells. The contribution of mTFP1 to the fluorescence signal increases, implying the cleavage of the biosensor. The effect of MMP-9 on the structure of biosensor has been followed with live cell imaging microscopy. 
         [0042]    This shows usefulness of the biosensor of the invention for investigation of the proteolytic activity of MMP-9 in vitro, as well as in vivo in living cells. 
       EXAMPLES 
     Materials Used in the Examples 
       [0043]    The genetically encoded FRET-based MMP-9 activity biosensor was assembled in the pDisplay plasmid (Clontech). The mTFP1 gene was amplified from the pmTFP1-N1 plasmid (Allele Biotech). The plasmid coding the Venus gene was provided by Jacek Jaworski (The International Institute of Cell Biology, Warsaw). 
         [0044]    Phusion Hot Start II Polymerase was purchased from Thermo Scientific (formerly Finnzymes). XmaI, SacII, Nhel, AflII, Agel, XbaI, Apal and BglII restriction enzymes were acquired in New England Biolabs and Thermo Scientific (formerly Fermentas). T4 DNA Polymerase required in the SLIC cloning was obtained from Thermo Scientific (formerly Fermentas). DMEM+GlutaMAX (High Glucose 4.5 g/L), Fetal Bovine Serum and penicillin/streptomycin mix were purchased from Sigma-Aldrich. Polyethylenoimine used for HEK293 cell line transfection was acquired from Fluka. Proteoextract Subcellular Proteome Kit was acquired from Calbiochem and EndoFree Plasmid DNA Maxi Kit from Qiagen. The α-GFP antibody was purchased from MBL (#498), the α-myc antibody from Santa Cruz Biotechnologies (#sc-40), the α-N-cadherin antibody from BD Biosciences (#610920), the α-hsp90 antibody from Stressgen (#SPS-771) and the α-histone H3 antibody from Abcam (#ab10799). 
         [0045]    Poly-L-lysine used to coat glass cover slips was purchased from Sigma-Aldrich. The auto-activating MMP-9 was designed and purified as described previously [Michaluk, et al., 2007]. The oligonucleotides were ordered either at Sigma Aldrich or Genomed. 
       Example 1 Construction of MMP-9 Activity Biosensor (Loop-Type Linker) 
       [0046]    The biosensor was cloned using the SLIC cloning methodology described in [Li, and Elledge, 2007]. The fluorescent protein genes were amplified with the Phusion Hot Start II High Fidelity Polymerase (Thermo Scientific) using the following primers: 
         [0000]    
       
         
               
               
             
           
               
                   
                 Venus1 
               
               
                   
                 forward primer: 
               
               
                   
                 (SEQ ID NO: 1) 
               
               
                   
                 CTGGGGCCCAGCCGGCCAGATCTCCCGGCATGGTGAG 
               
               
                   
                 CAAGGGCGAGGA 
               
               
                   
                   
               
               
                   
                 reverse primer: 
               
               
                   
                 (SEQ ID NO: 2) 
               
               
                   
                 TCCTCGCCCTTGCTCACCAT GCTAGC CTTGTACAGCT 
               
               
                   
                 CGTCCATGC 
               
               
                   
                   
               
               
                   
                 Venus2 
               
               
                   
                 forward primer: 
               
               
                   
                 (SEQ ID NO: 3) 
               
               
                   
                 GCATGGACGAGCTGTACAAG GCTAGC ATGGTGAGCAA 
               
               
                   
                 GGGCGAGGA 
               
               
                   
                   
               
               
                   
                 reverse primer: 
               
               
                   
                 (SEQ ID NO: 4) 
               
               
                   
                 TCCTCGCCCTTGCTCACCAT CTTAAG CTTGTACAGCT 
               
               
                   
                 CGTCCATGC 
               
               
                   
                   
               
               
                   
                 mTFP1 
               
               
                   
                 forward primer: 
               
               
                   
                 (SEQ ID NO: 5) 
               
               
                   
                 GCATGGACGAGCTGTACAAG CTTAAG ATGGTGAGCAA 
               
               
                   
                 GGGCGAGGA 
               
               
                   
                   
               
               
                   
                 reverse primer: 
               
               
                   
                 (SEQ ID NO: 6) 
               
               
                   
                 AGATGAGTTTTTGTTCGTCGACCTGCAGCCGCACTTG 
               
               
                   
                 TACAGCTCGTCCATGC. 
               
             
          
         
       
     
         [0047]    The pDisplay plasmid was cleaved with Xmal and Sacll enzymes to generate single stranded ends. Proper assembly of the Venus-Venus-mTFP1 tandem construct was confirmed using restriction enzyme analysis and sequencing. 
         [0048]    Two restriction enzyme sites were introduced into the tandem construct: Nhel site separated the Venus 1 and Venus2 fluorescent proteins, while AflII was cloned between the Venus2 and mTFP1 genes (sites marked in bold in primer sequences). Two oligonucleotides were cloned into these sites, each one coding a peptide linker designed to be flexible and provide spatial separation between the fluorescent proteins. Oligonucleotides were composed of repeated segments separated with restriction enzyme cleavage sites (Agel and Xbal) to enable a rapid and simple adjustment of their length. An oligonucleotide (coding linker LN1) with the following sequence was cloned into the AflII site: 
         [0000]                            (SEQ ID NO: 7)           CTTAAGGGATCC CCCCGCTCTCTCTCT AAGCTTAAA-           (GGAGGA ACCGGT GGAACT) 8 -CTTAAG.            
The Agel restriction site is in bold font, the sequence coding the MMP-9 cleavage site was underlined. The following oligonucleotide (coding linker LN2) was cloned into the NheI site:
 
         [0000]    
       
         
               
               
             
           
               
                   
                 (SEQ ID NO: 8) 
               
               
                   
                 GCTAGCGGTGGTAGCGGTGGTAGCGGtGCTAGT-(GG 
               
               
                   
                 TGGTTCTGGT TCTAGA ) 8 -GCTAGC. 
               
             
          
         
       
     
         [0049]    The XbaI restriction site is in bold. The biosensor constructed in the above fashion carried the MMP-9 cleavage site within an unstructured loop, hence the name: biosensor with a loop-like linker. 
       Example 2 Construction of MMP-9 Activity Biosensor (α-Helical Cleavage Site) 
       [0050]    To construct a variant MMP-9 activity biosensor with the MMP-9 cleavage site located within α-helical structure, the linker between Venus and mTFP1 in biosensor with a loop-like linker was cut out with the AflII enzyme and replaced with the following oligonucleotide (SEQ NO ID: 9): 
         [0000]                            CTTAAGGAGGAGGAGATCAGAGAGGCCTTCAGAGTGT                       TCCCCAGAAGCCTGAGCCTGAGACACGTGATGACCAA                       CCTGCTTAAG            
It encodes the following peptide (+1 Open Reading Frame):
 
EEEIREAFRVFPRSLSLRHVMTNL (SEQ ID NO: 10), where sequences given in bold represent α-helices.
 
       Example 3 Construction of Membrane Anchored mTFP1 Required for FLIM 
       [0051]    The membrane anchored mTFP1 was constructed in the pDisplay plasmid. The mTFP1 gene was amplified with the following primers— 
         [0000]    
       
         
               
               
             
           
               
                   
                 forward primer: 
               
               
                   
                 (SEQ ID NO: 11) 
               
               
                   
                 AAGAAC GGGCCC ATGGTGAGCAAGGGCGAGG 
               
               
                   
                   
               
               
                   
                 reverse primer: 
               
               
                   
                 (SEQ ID NO: 12) 
               
               
                   
                 AAGAAC AGATCT CTTGTACAGCTCGTCCATGC. 
               
             
          
         
       
     
         [0052]    Given in bold are restriction sites—Apal in forward primer and BglII in reverse primer, respectively. The PCR product was cloned between these sites into the pDisplay plasmid. 
       Example 4 Construction of MMP-9 Activity Biosensors with Varying Linker Lengths 
       [0053]    A series of variant biosensors with altered linker lengths were generated to perform FRET efficiency optimization for the biosensor. The plasmid coding the biosensor with full length linkers was subjected to partial cleavage with either Agel or Xbal enzymes, religation and transformation into  E. coli . Restriction enzyme cleavage reactions were performed with increasing amounts of enzyme. The LN1 linker was cleaved for 2 h with an amount of Agel sufficient to cut from 1 to 4 of its restriction sites—0.85 U, 1 U, 1.5 U, 2 U, 2.5 U, 3 U, 3.5 U of Agel were used. Similarly the LN2 linker was cleaved with 6 U, 8 U, 10 U, 12 U, 14 U, 16 U 18 U or 20 U for 2 h, to cleave the plasmid in 1 to 4 Xbal sites. Clones were analyzed using PCR to determine the linker length within the biosensors and sequenced. 
         [0054]    The following biosensors were received: 1-1, 2-1, 3-1, 4-1, 5-1, 6-1, 7-1, 8-1, 1-2, 2-2, 8-2, 1-3, 2-3, 3-3, 4-3, 5-3, 6-3, 7-3, 8-3, 1-4, 2-4, 8-4, 1-5, 2-5, 8-5, 1-6, 2-6, 8-6, 1-7, 8-7, 1-8, 2-8, 3-8, 4-8, 5-8, 6-8, 7-8, 8-8, wherein the first digit in the number pair designating the biosensor variant indicates a number of GGSGGR hexapeptide repeats in LN2 linker and the second digit in the number pairs indicates a number of GGTGGT hexapeptide repeats in LN1 linker. 
       Example 5 Transfection of MMP-9 Activity Biosensors into HEK293 Cells 
       [0055]    Routine FRET optimization and testing of the sensor were performed in the HEK293 cell line. Cells were cultured in DMEM (4.5 g/L glucose)+10% FBS+1% P/S in 37° C., 5% CO 2 . Plasmids coding the sensors (identified in the proceeding Examples) were purified with the Qiagen Endo Free Plasmid Maxi Kit. DNA to be transfected was mixed with pure DMEM and polyethylenoimine (PEI) (5 μg/μL), left for 10 minutes at room temperature, and then transferred to cell culture. Cells were incubated with DNA-PEI complexes for 4 h, then the medium was replaced with a fresh one. Cells intended to be imaged on confocal microscope were cultured on glass cover slips coated with poly-L-lysine. 
       Example 6 AP/FLIM Analysis of the Cells Expressing MMP-9 Activity Biosensors 
       [0056]    Two days post transfection the cells obtained in Example 5 were fixed with 4% PFA, 3% sucrose in PBS and microscope slides were prepared. Acceptor photobleaching (AP) experiments were performed on Leica SP5 microscope with 63×NA (1.4) oil immersion objective. Images were acquired at 1024×1024 pixels. mTFP1 was imaged with the 458 nm line of an argon laser set to 20%. FRET efficiency of the sensors was determined by AP of the Venus with high power 514 nm laser and measuring the increase in the intensity of the fluorescence of mTFP1. Sensors determined to have highest FRET efficiency were further analyzed with the Fluorescence Lifetime Imaging Microscopy (FLIM) on Leica SP2 microscope. 
         [0057]    The apparent FRET efficiency value of variant sensors from AP data was calculated using the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Ef 
                       D 
                     
                     = 
                     
                       1 
                       - 
                       
                         ( 
                         
                           
                             F 
                             DA 
                           
                           
                             F 
                             D 
                           
                         
                         ) 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where f D  is the fraction of donor participating in the FRET complex, F DA  and F D  are the background subtracted and acquisition bleaching corrected pre- and post-bleach mTFP1 fluorescence intensities, respectively. The acquisition bleaching corrected post-bleach mTFP1 intensities were calculated as 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       F 
                       D 
                     
                     = 
                     
                       
                         F 
                         D 
                         
                           B 
                           · 
                           post 
                         
                       
                       + 
                       
                         
                           ( 
                           
                             
                               
                                 F 
                                 D 
                                 
                                   R 
                                   · 
                                   pre 
                                 
                               
                               - 
                               
                                 F 
                                 D 
                                 
                                   R 
                                   · 
                                   post 
                                 
                               
                             
                             
                               F 
                               D 
                               
                                 R 
                                 · 
                                 pre 
                               
                             
                           
                           ) 
                         
                          
                         
                           F 
                           D 
                           
                             B 
                             · 
                             pre 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where F D   B  and F D   R  refer to mTFP1 intensities of the bleach and reference region of interest, and pre and post refer to pre-bleach and post-bleach measurements. 
         [0058]    FRET efficiency values from FLIM data were calculated with the following equation 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Ef 
                       D 
                     
                     = 
                     
                       
                         ( 
                         
                           1 
                           - 
                           
                             
                               τ 
                               D 
                             
                             
                               τ 
                               DA 
                             
                           
                         
                         ) 
                       
                        
                       
                         
                           A 
                           DA 
                         
                         
                           
                             A 
                             DA 
                           
                           + 
                           
                             A 
                             D 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where τ D  is the lifetime of the donor in the absence of the acceptor (in our case the membrane anchored mTFP1) and τ DA  is the lifetime of the FRET-based MMP-9 activity biosensor and A DA  and A D  represent the amplitude of individual decay components [Zeug, et al., 2012]. Error values were estimated using the Gaussian noise propagation equation 
         [0000]    
       
         
           
             
               
                 
                   stdE 
                   = 
                   
                     
                       
                         
                           
                             
                               ( 
                               
                                 
                                   ∂ 
                                   E 
                                 
                                 
                                   ∂ 
                                   
                                     τ 
                                     D 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
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                             Δτ 
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                             2 
                           
                         
                         + 
                         
                           
                             
                               ( 
                               
                                 
                                   ∂ 
                                   E 
                                 
                                 
                                   ∂ 
                                   
                                     τ 
                                     DA 
                                   
                                 
                               
                               ) 
                             
                             2 
                           
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                             Δτ 
                             DA 
                             2 
                           
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
       Example 7 Fluorescence Emission Spectra Collection for Cells Transfected with MMP-9 Activity Biosensors 
       [0059]    Lambda stack acquisition was performed on Zeiss LSM780 microscope equipped with 63×NA (1.4) oil-immersion objective at 1024×1024 pixels. The 458 nm line of an argon laser was used for excitation and 32 lambda channels were acquired, at 9 nm steps. Acquired lambda stacks were analysed with Fiji ImageJ software by measuring the average brightness of the plasma membrane in each channel. Recovered sensor spectra were normalized by having the area under the spectrum plot equaling 1. 
       Example 8 HEK293 Cell Fractioning 
       [0060]    Cell fractioning experiments were performed using the Calbiochem ProteoExtract Subcellular Proteome Extraction kit. Sensor was detected on Western Blot using the anti-myc antibody. Quality of the cell fractioning was tested on Western Blot with the following antibodies: anti-hsp90, anti-N-cadherin and anti-histone H3. 
       Example 9 In Vitro Cleavage of the MMP-9 Activity Biosensors 
       [0061]    Two days post-transfection HEK293 cells were washed one with PBS, scraped from the plate and lysed for 1 h at 4° C. with the following buffer: 50 mM Tris-CI pH 7.5, 1% Triton X-100, 10 mM CaCl 2 , 0.02% NaN 3 , 1 μM ZnCl 2 . The lysis was performed without protease inhibitors since it was feared that they might block the activity of our auto-activating MMP-9. The lysate was then centrifuged at 13400 rpm for 15′ at 4° C. to remove cell debris. Equal amount of the cleared lysate were used in the subsequent reactions. Either 400 ng (final concentration 10 μg/mL), 1.2 μg (final concentration 30 μg/mL) of auto-activating MMP-9 or 400 ng (final concentration 10 μg/mL) of inactive MMP-9 were added to the reactions. GM6001 inhibitor was used in 25 μM final concentration. Reactions were stopped at either 30′, 1 h, 4 h or after overnight incubation at 37° C. with the addition of SDS-PAGE Sample Buffer and heating to 100° C. for 10 minutes. Sensor was detected on Western Blot with the anti-GFP antibody. 
       Example 10 Cleavage of the MMP-9 Activity Biosensors in the Cell Culture 
       [0062]    Two days post-transfection with the MMP-9 activity biosensors, the culture medium was replaced with pure DMEM. Cells were incubated for 30 min. with 400 ng of auto-activating MMP-9 (final concentration—800 ng/mL) and fixed with 4% PFA, 3% sucrose in PBS. Lambda stacks were acquired as previously described. 
       Example 12 Live Imaging—Ratiometric Analysis of Cells Transfected with MMP-9 Activity Bio Sensors 
       [0063]    The HEK293 cell line was cultured on Glass Bottom Microwell Dishes (MatTek Corporation). The cells were transfected with a plasmid coding the biosensor with α-helical liner. Two days post-transfection the cells were transferred to a Zeiss LSM780 microscope fitted with incubator and imaged using a water-immersion 40× objective. A single optical slice of the cells was captured at the 1024×1024 pixel resolution every 30 s with linear unmixing of the donor and acceptor fluorescence spectra performed in real time. Acquisition was performed for 30 min. 5 min after the start of image acquisition the cells were either mock treated with pure DMEM, auto-activating MMP-9 diluted in DMEM (final concentration—460 ng/mL) or inactive MMP-9 similarly diluted in DMEM to the same final concentration. Data analysis was performed in the custom written software under Matlab suite. The Venus/mTFP1 ratio was calculated for each pixel and plotted against the time elapsed from the start of the experiment. 
       Results 
     I. FRET Efficiency of the Biosensor (AP and FLIM Study) 
       [0064]    The introduction of linkers with adjustable lengths allowed a rapid formation of 38 biosensor variants. Biosensors of the invention with the highest FRET efficiency were identified with the AP technique ( FIG. 1B ) and analyzed using FLIM to confirm AP-based FRET efficiency calculations. 
         [0065]    Table 1 presents FRET Efficiency values calculated from FLIM data for the biosensors variants with the highest FRET efficiencies in AP experiments. Error values were estimated using the Gaussian noise propagation equation. The naming scheme of the variants is as follows: X-Y, where X—a number of hexapeptide repeats between both Venus FP (linker LN2), Y—a number of hexapeptide repeats between Venus FP and mTFP1 (linker LN1). 
         [0000]                                              TABLE 1                   FRET Efficiency of variant       FRET Efficiency of variant MMP-9 activity       biosensors with a loop-like linker - FLIM       Sensor variant            3-1   4-1   7-1   8-1   8-3   2-8               0.22 ±   0.22 ±   0.23 ±   0.19 ± 0.05   0.20 ± 0.05   0.22 ± 0.05       0.05   0.05   0.05                    
A biosensor (named 7-1 in Table 1) with a long linker between the two Venus FP (seven repeats GGTGGTTCTGGTTCTAGA (SEQ ID NO: 13) in its DNA sequence) and a short linker between the second Venus and mTFP1 (one GGAGGAACCGGTGGAACT repeat (SEQ ID NO: 14) in its DNA sequence) has the highest FRET efficiency of all obtained biosensor loop-like linker variants and the FRET efficiency was found to be E=0.20±0.03 (standard deviation value)— FIG. 2A . The biosensor with α-helical linker has a higher acquisition-bleaching corrected effective FRET efficiency 0.26±0.02 in comparison to the biosensor with the loop-like linker— FIG. 2B . Given the higher FRET efficiency exhibited by the biosensor with α-helical linker, it was used in further studies.
 
       II. Cellular Membrane Localization 
       [0066]    The MMP-9 activity biosensor of the invention localizes at the cellular membrane. This was confirmed by direct visualization of the biosensor in living HEK293 cells ( FIG. 1C ) and through cell fractionation followed by western blot analysis of collected fractions ( FIG. 1D ). Fractionation confirms that the vast majority (87%±6%) of the biosensor localizes in membranes with a small percentage in the cytoplasm. No biosensor was observed in the nucleus. Control western blots confirmed the purity of collected fractions. 
       III. In Vitro Biosensor Cleavage 
       [0067]    The MMP-9 activity biosensor is cleaved in vitro by a human auto-activating MMP-9 ( FIG. 3 ). Significantly more enzyme was used (standard concentration of auto-activating MMP-9 in experiments is 400 ng/mL [Michaluk et al., 2009]) in these reactions to make sure that the entire biosensor pool was cleaved. 
         [0068]    The biosensor is already partially cleaved (14.2±0.4%—see  FIG. 3B , control lanes; value normalized to full length biosensor intensity in control lanes) in transfected but otherwise untreated HEK293 cell line, which can be readily seen on the Western blot ( FIG. 3 ). The in vitro cleavage of the biosensor with α-helical linker is not due to spontaneous degradation of the protein, though a slight increase in the amount of a cleaved form of the biosensor is observed over time ( FIG. 3 ). Inactive human MMP-9 does not cleave the biosensor ( FIG. 3 ). The cleavage can be blocked by the addition of a broad spectrum GM6001 matrix metalloproteinase inhibitor to the final concentration of 25 μM ( FIG. 3 ). 
       IV. Biosensor Cleavage in the HEK293 Cell Culture—Fluorescence Emission Spectra of Fixed Cells 
       [0069]    Analysis of the fluorescence emission spectra collected from HEK293 cells incubated with the auto-activating MMP-9 for 30 minutes confirms that the biosensor is being cleaved in the cellular membrane ( FIG. 4 ). The biosensor cleavage can be observed as a change in the emission spectrum of the biosensor within the membrane. The treatment with auto-activating MMP-9 results in a shift of the spectrum towards shorter wavelengths, a decrease in the contribution of Venus to the fluorescence signal (acceptor peak) and a corresponding increase in mTFP1 contribution (donor peak). 
       V. Biosensor Cleavage in the HEK293 Cell Culture—Live Cell Imaging 
       [0070]    The cleavage of the biosensor can be observed in live imaging of HEK293 cells ( FIG. 5 ) as a decrease of the Venus to mTFP1 fluorescence intensity ratio ( FIG. 5 ). The addition of auto-activating MMP-9 to the culture medium results in a decrease in the ratio whereas mock treatment with pure medium or a treatment with inactive MMP-9 results in a slight, gradual increase ( FIG. 5B ). 
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