In situ quantitative measurements on MMP-9 activity in single living cells by single molecule fluorescence correlation spectroscopy

Matrix metalloproteinase-9 (MMP-9) plays an important role in tumor progression. It is of great significance to establish a sensitive in situ assay strategy for MMP-9 activity in single living cells. Here a novel in situ single molecule spectroscopy method based on the fluorescence correlation spectroscopy (FCS) technique was proposed for measuring the MMP-9 activity at different locations within single living cells, using a fluorescent specific peptide and a reference dye as dual probes. The measurement principle is based on the decrease of the ratiometric translational diffusion time of dual probes in the detection volume due to the peptide cleavage caused by MMP-9. The peptide probe was designed to be composed of an MMP-9 cleavage and cell-penetrating peptide sequence that was labeled with a fluorophore and conjugated with a streptavidin (SAV) molecule. The ratiometric translational diffusion time was used as the measurement parameter to eliminate the effect of intracellular uncertain viscosity. The linear relationship between the ratiometric diffusion time and MMP-9 activity was established, and applied to the determination of enzymatic activity in cell lysates as well as the evaluation of the inhibitory effects of different inhibitors on MMP-9. More importantly, the method was successfully used to dynamically determine MMP-9 activity in single living cells or under the stimulation with phorbol 12-myristate 13-acetate (PMA) and inhibitors.

Analysis of protein phosphorylation in solution and in cells by using an ATP analogue in combination with fluorescence techniques

Protein phosphorylation is a very important mechanism for regulating and controlling the activity and function of proteins, and is closely associated with signal transduction, gene expression, cell cycle and other life activities in organisms. In this paper, we proposed a new strategy for studying protein phosphorylation in living cells by combining fluorescence resonance energy transfer (FRET) with a small molecule adenosine 5'-triphosphate (ATP) analogue. We synthesized a new ATP analogue functionalized by norbornene (ATP-NB), and a tetrazine modified fluorescent probe Cyanine3 (TZ-Cy3). Based on the inverse electron demand Diels-Alder (D-A) reaction, ATP-NB phosphorylated proteins in solution and in living cells were in situ labelled with TZ-Cy3. By combining FRET with fluorescence correlation spectroscopy (FRET-FCS) and imaging technology, we established an efficient method for studying the phosphorylation of proteins in solution and in living cells using an ATP analogue instead of natural ATP. We studied the effects of phosphatase inhibitors on the phosphorylation of proteins in living cells. Our results documented that ATP-NB is a small molecule ATP analogue with hydrophobicity, which can penetrate cells and efficiently phosphorylate proteins in living cells. This strategy is well suitable for in situ study of protein phosphorylation in living cells.

Simultaneously monitoring endogenous MAPK members in single living cells by multi-channel fluorescence correlation spectroscopy

The mitogen-activated protein kinase (MAPK) pathway is a major module for cellular signal transduction. The dysregulation of the MAPK pathway has been involved in the pathogenesis of multiple diseases ranging from cancers to chronic inflammations. So far, we have not fully understood the influences of external factors and signaling networks on the MAPK pathway due to the lack of in situ methods for simultaneous detection of multiple kinases in the pathway in living cells. Herein, we present a new strategy for in situ and simultaneously monitoring MAPK pathway kinases in single living cells combining multi-channel fluorescence correlation spectroscopy (FCS) with affinity fluorescent probes. We chose rapidly growing fibrosarcoma kinase (RAF), mitogen-activated protein kinase (MEK), and extracellular signal-regulated kinase (ERK) as representative members in the MAPK pathway. We designed and synthesized three fluorescent affinity probes and experimental results demonstrated that the three probes specifically targeted endogenous BRAF, MEK1/2, and ERK1/2 in living cells. Based on the multi-channel FCS system, we studied the influences of biological substances, drugs and oxidative stress on the activities of endogenous MAPK kinases and the cross-talk between the MAPK and PI3K-mTOR pathways. We have found that serum, sorafenib, and hydrogen peroxide can regulate multiple MAPK kinases and the effects of external stimuli can transmit to the MAPK pathway; furthermore, we have observed that the MAPK pathway can be activated by modulating the PI3K-mTOR pathway. Our results illustrated the complexity of a cellular signal network and the necessity of in situ and simultaneous determination of biomolecules in living cells.

In situ determination of secretory kinase Fam20C from living cells using fluorescence correlation spectroscopy

Secretory proteins constitute a biologically crucial subset of proteins for regulation of some pathological and physiological processes, and they have become very important biomarkers in clinical diagnosis and therapeutic targets. So far, secretory protein functions and mechanisms have not been fully understood due to methodological limitations in detection of low-abundance proteins against medium background. Here, we propose a strategy to determine secretory protein from living cells in situ using fluorescence correlation spectroscopy (FCS). In this study, the recombinant protein Fam20C with SNAP-tag was used as a model protein, and O⁶-benzylguanine (BG) derivatives bearing fluorescent dye as probes. We synthesized three fluorescent probes and investigated their fluorescent properties and diffusion behaviors in solution, and found the probe BG-Bodipy-561 more suitable for in situ labeling of Fam20C. We confirmed the specific binding of the probe to the target protein by combining FCS and in-gel fluorescence scanning methods. We studied the effects of some factors of the secretory Fam20C, and found that RNA interference significantly inhibited the synthesis of secretory fused Fam20C, and myriocin had no significant effect on the expression of secretory Fam20C, which indirectly illustrated that sphingolipid signaling can regulate the Fam20C activity. We believe that FCS is a very promising method to analyze secretory proteins from living cells in situ.