R e s e a R c h a R t i c l e4 5 3 0 jci.org Volume 125 Number 12 December 2015 ing activation of HER2 or MET signaling, mutation of PIK3CA and BRAF, or expression status of PTEN, but retrospective analyses revealed inconsistent and controversial findings (16). So far, the most accepted predictive marker for poor cetuximab response is mutant KRAS status, due to its association with poor survival rate under cetuximab treatment in colorectal cancer clinical trials (17)(18)(19)(20). Therefore, American Society of Clinical Oncology recommended cetuximab treatment for only patients with WT KRAS (21). However, there is increasing evidence showing that WT KRAS is not sufficient to confer sensitivity to cetuximab (22)(23)(24), and some patients with mutant KRAS are still sensitive to cetuximab (16,(25)(26)(27)(28). These findings suggest that further investigation into the underlying mechanisms of cetuximab resistance and identification of a better predictor for cetuximab response are ing of chemotherapeutic agents have improved the response and survival rate of colorectal patients. Currently, rational targeting of molecular signaling pathways that are involved in the etiology of malignancies is one of the most promising strategies in novel anticancer drug development (13). Owing to the role of EGFR in tumorigenesis, new classes of drugs that target EGFR are among the most clinically advanced molecular-targeted therapies. Although EGFR tyrosine kinase inhibitors combined with chemotherapy presented severe toxicity and limited effect (14), the combination of EGFR monoclonal antibody, such as cetuximab and panitumumab, with chemotherapy has shown efficacy in colorectal cancer treatment (15). Unfortunately, resistance to EGFR-targeted therapy has recently been observed. Many mechanisms have been proposed to explain the poor response to cetuximab, includ-

A simple microfluidic 3D hydrodynamic flow focusing device has been developed and demonstrated quantitative determinations of quantum dot 525 with antibody (QD525-antibody) and hemagglutinin epitope tagged MAX (HA-MAX) protein concentrations. This device had a step depth cross junction structure at a hydrodynamic flow focusing point at which the analyte stream was flowed into a main detection channel and pinched not only horizontally but also vertically by two sheath streams. As a result, a triangular cross-sectional flow profile of the analyte stream was formed and the laser was focused on the top of the triangular shaped analyte stream. Since the detection volume was smaller than the radius of laser spot, a photon burst histogram showed Gaussian distribution, which was necessary for the quantitative analysis of protein concentration. By using this approach, a linear concentration curve of QD525-antibody down to 10 pM was demonstrated. In addition, the concentration of HA-MAX protein in HEK293 cell lysate was determined as 0.283 ± 0.015 nM. This approach requires for only 1 min determining protein concentration. As the best of our knowledge, this is the first time to determinate protein concentration by using single molecule detection techniques.

The understanding of protein interaction dynamics is important for signal transduction research but current available techniques prove difficult in addressing this issue. Thus, using the microfluidic approach, we developed a digital protein analytical platform and methodology named MAPS (Microfluidic system Analyzing Protein in Single complex) that can measure the amount of target proteins and protein complexes at the digitally single molecule resolution. By counting protein events individually, this system can provide rough protein interaction ratios which will be critical for understanding signal transduction dynamics. In addition, this system only requires less than an hour to characterize the target protein sample, which is much quicker than conventional approaches. As a proof of concept, we have determined the interaction ratios of oncogenic signaling protein complexes EGFR/Src and EGFR/STAT3 before and after EGF ligand stimulation. To the best of our knowledge, this is the first time that the interaction ratio between EGFR and its downstream proteins has been characterized. The information from MAPS will be critical for the study of protein signal transduction quantitation and dynamics.

Signal transduction is a dynamic process that regulates cellular functions through multiple types of biomolecular interactions, such as the interactions between proteins and between proteins and nucleic acids. However, the techniques currently available for identifying protein-protein or protein–nucleic acid complexes typically provide information about the overall population of signaling complexes in a sample instead of information about the individual signaling complexes therein. We developed a technique called “microchannel for multiparameter analysis of proteins in a single complex” (mMAPS) that simultaneously detected individual target proteins either singly or in a multicomponent complex in cell or tissue lysates. We detected the target proteins labeled with fluorophores by flow proteometry, which provided quantified data in the form of multidimensional fluorescence plots. Using mMAPS, we quantified individual complexes of epidermal growth factor (EGF) with its receptor EGFR, EGFR with signal transducer and activator of transcription 3 (STAT3), and STAT3 with the acetylase p300 and DNA in lysates from cultured cells with and without treatment with EGF, as well as in lysates from tumor xenograft tissue. Consistent with the ability of this method to reveal the dynamics of signaling protein interactions, we observed that cells treated with EGF induced the interaction of EGF with EGFR and the autophosphorylation of EGFR, but this interaction decreased with longer treatment time. Thus, we expect that this technique may reveal new aspects of molecular interaction dynamics.

We have developed a microfluidics based platform and methodology named MAPS (microfluidic system for analyzing proteins in single complex) for detecting two protein interactions rapidly using a single fluorophore. Target proteins were labelled with Quantum dot 525 (QD525) via specific polyclonal antibodies, and were transported through the microfluidic channel subsequently, where the 375 nm excitation laser light was focused to form a detection volume. Photon bursts from target proteins passing through the detection volume were recorded and their photon burst histograms were plotted which demonstrated roughly the specific protein interaction ratio based on their population and statistical behavior. As a proof of concept, Src/STAT3 protein complex interaction ratios with and without EGF stimulation were obtained by MAPS within 1 h and the results were well matched with the one obtained by the conventional immunoprecipitation/ Western blot (IP/WB).

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Signal transduction is essential for maintaining cells’ normal physiological functions, and deregulation of signaling can lead to diseases such as diabetes and cancers. Some of the major players in signal delivery are molecular complexes composed of proteins and nucleic acids. This unit describes a technique called microchannel for Multi-parameter Analysis of Proteins in a Single-complex or mMAPS for analyzing and quantifying target signaling complexes individually. mMAPS is a flow-proteometric-based system that allows detection of individual proteins or complexes flowing through a microfluidic channel. Specific target proteins and nucleic acids labeled by fluorescent tags are harvested from tissue samples or cultured cells for analysis by the mMAPS system. Overall, mMAPS enables both detection of multiple components within a single complex and direct quantification of different populations of molecular complexes in one setting in a short timeframe, requiring very low sample input.