Professional Education


  • Master of Science, Indian Institute of Technology (2008)
  • Doctor of Philosophy, Carnegie Mellon University (2014)

Stanford Advisors


All Publications


  • Super-resolution Imaging of Live Bacteria Cells Using a Genetically Directed, Highly Photostable Fluoromodule. Journal of the American Chemical Society Saurabh, S., Perez, A. M., Comerci, C. J., Shapiro, L., Moerner, W. E. 2016; 138 (33): 10398-10401

    Abstract

    The rapid development in fluorescence microscopy and imaging techniques has greatly benefited our understanding of the mechanisms governing cellular processes at the molecular level. In particular, super-resolution microscopy methods overcome the diffraction limit to observe nanoscale cellular structures with unprecedented detail, and single-molecule tracking provides precise dynamic information about the motions of labeled proteins and oligonucleotides. Enhanced photostability of fluorescent labels (i.e., maximum emitted photons before photobleaching) is a critical requirement for achieving the ultimate spatio-temporal resolution with either method. While super-resolution imaging has greatly benefited from highly photostable fluorophores, a shortage of photostable fluorescent labels for bacteria has limited its use in these small but relevant organisms. In this study, we report the use of a highly photostable fluoromodule, dL5, to genetically label proteins in the Gram-negative bacterium Caulobacter crescentus, enabling long-time-scale protein tracking and super-resolution microscopy. dL5 imaging relies on the activation of the fluorogen Malachite Green (MG) and can be used to label proteins sparsely, enabling single-protein detection in live bacteria without initial bleaching steps. dL5-MG complexes emit 2-fold more photons before photobleaching compared to organic dyes such as Cy5 and Alexa 647 in vitro, and 5-fold more photons compared to eYFP in vivo. We imaged fusions of dL5 to three different proteins in live Caulobacter cells using stimulated emission depletion microscopy, yielding a 4-fold resolution enhancement compared to diffraction-limited imaging. Importantly, dL5 fusions to an intermediate filament protein CreS are significantly less perturbative compared to traditional fluorescent protein fusions. To the best of our knowledge, this is the first demonstration of the use of fluorogen activating proteins for super-resolution imaging in live bacterial cells.

    View details for DOI 10.1021/jacs.6b05943

    View details for PubMedID 27479076

    View details for PubMedCentralID PMC4996739

  • Kinetically Tunable Photostability of Fluorogen-Activating Peptide-Fluorogen Complexes CHEMPHYSCHEM Saurabh, S., Zhang, M., Mann, V. R., Costello, A. M., Bruchez, M. P. 2015; 16 (14): 2974–80

    Abstract

    Ease of genetic encoding, labeling specificity, and high photostability are the most sought after qualities in a fluorophore for biological detection. Furthermore, many applications can gain from the fluorogenic nature of fluoromodules and the ability to turn on the same fluoromodules multiple times. Fluorogen-activating peptides (FAPs) bind noncovalently to their cognate fluorogens and exhibit enhanced photostability. Herein, the photostabilities of malachite green (MG)-binding and thiazole-orange-binding FAPs are compared under limiting- and excess-fluorogen conditions to establish distinct mechanisms for photostability that correspond to the dissociation rate of the FAP-fluorogen complex. FAPs with slow dissociation show evidence of dye encapsulation and protection from photo or environmental degradation and single-step bleaching at the single molecule level, whereas those with rapid dissociation show repeated cycles of binding and enhanced photostability by exchange of bleached fluorogen with a new dye. A combination of generalizable selection pressure based on bleaching, flow cytometry, and site-specific amino acid mutagenesis is used to obtain a modified FAP with enhanced photostability, due to rapid dissociation of the MG fluorogen. These studies shed light on the basic mechanisms by which noncovalent association can effect photostable labeling, and demonstrate novel reagents for photostable and intermittent labeling of biological targets.

    View details for DOI 10.1002/cphc.201500587

    View details for Web of Science ID 000362834600008

    View details for PubMedID 26310607

  • Genetically targeted fluorogenic macromolecules for subcellular imaging and cellular perturbation BIOMATERIALS Magenau, A. D., Saurabh, S., Andreko, S. K., Telmer, C. A., Schmidt, B. F., Waggoner, A. S., Bruchez, M. P. 2015; 66: 1–8

    Abstract

    The alteration of cellular functions by anchoring macromolecules to specified organelles may reveal a new area of therapeutic potential and clinical treatment. In this work, a unique phenotype was evoked by influencing cellular behavior through the modification of subcellular structures with genetically targetable macromolecules. These fluorogen-functionalized polymers, prepared via controlled radical polymerization, were capable of exclusively decorating actin, cytoplasmic, or nuclear compartments of living cells expressing localized fluorgen-activating proteins. The macromolecular fluorogens were optimized by establishing critical polymer architecture-biophysical property relationships which impacted binding rates, binding affinities, and the level of internalization. Specific labeling of subcellular structures was realized at nanomolar concentrations of polymer, in the absence of membrane permeabilization or transduction domains, and fluorogen-modified polymers were found to bind to protein intact after delivery to the cytosol. Cellular motility was found to be dependent on binding of macromolecular fluorogens to actin structures causing rapid cellular ruffling without migration.

    View details for DOI 10.1016/j.biomaterials.2015.07.002

    View details for Web of Science ID 000359328200001

    View details for PubMedID 26183934

    View details for PubMedCentralID PMC4522379

  • Multiplexed Modular Genetic Targeting of Quantum Dots ACS NANO Saurabh, S., Beck, L. E., Maji, S., Baty, C. J., Wang, Y., Yan, Q., Watkins, S. C., Bruchez, M. P. 2014; 8 (11): 11138–46

    Abstract

    While DNA-directed nanotechnology is now a well-established platform for bioinspired nanoscale assembly in vitro, the direct targeting of various nanomaterials in living biological systems remains a significant challenge. Hybrid biological systems with integrated and targeted nanomaterials may have interesting and exploitable properties, so methods for targeting various nanomaterials to precise biological locations are required. Fluorescence imaging has benefited from the use of nanoparticles with superior optical properties compared to fluorescent organic dyes or fluorescent proteins. While single-particle tracking (SPT) in living cells with genetically encoded proteins is limited to very short trajectories, the high photon output of genetically targeted and multiplexed quantum dots (QDs) would enable long-trajectory analysis of multiple proteins. However, challenges with genetic targeting of QDs limit their application in these experiments. In this report, we establish a modular method for targeting QD nanoparticles selectively to multiple genetically encoded tags by precomplexing QD-streptavidin conjugates with cognate biotinylated hapten molecules. This approach enables labeling and SPT of multiple genetically encoded proteins on living cells at high speed and can label expressed proteins in the cytosol upon microinjection into living cells. While we demonstrate labeling with three distinct QD conjugates, the approach can be extended to other specific hapten-affinity molecule interactions and alternative nanoparticles, enabling precise directed targeting of nanoparticles in living biological systems.

    View details for DOI 10.1021/nn5044367

    View details for Web of Science ID 000345553000015

    View details for PubMedID 25380615

    View details for PubMedCentralID PMC4246007

  • Molecular crowding shapes gene expression in synthetic cellular nanosystems NATURE NANOTECHNOLOGY Tan, C., Saurabh, S., Bruchez, M. P., Schwartz, R., LeDuc, P. 2013; 8 (8): 602–8

    Abstract

    The integration of synthetic and cell-free biology has made tremendous strides towards creating artificial cellular nanosystems using concepts from solution-based chemistry, where only the concentrations of reacting species modulate gene expression rates. However, it is known that macromolecular crowding, a key feature in natural cells, can dramatically influence biochemical kinetics via volume exclusion effects, which reduce diffusion rates and enhance binding rates of macromolecules. Here, we demonstrate that macromolecular crowding can increase the robustness of gene expression by integrating synthetic cellular components of biological circuits and artificial cellular nanosystems. Furthermore, we reveal how ubiquitous cellular modules, including genetic components, a negative feedback loop and the size of the crowding molecules can fine-tune gene circuit response to molecular crowding. By bridging a key gap between artificial and living cells, our work has implications for efficient and robust control of both synthetic and natural cellular circuits.

    View details for DOI 10.1038/NNANO.2013.132

    View details for Web of Science ID 000322713500017

    View details for PubMedID 23851358

    View details for PubMedCentralID PMC3951305

  • Identification of PAmKate as a Red Photoactivatable Fluorescent Protein for Cryogenic Super-Resolution Imaging. Journal of the American Chemical Society Dahlberg, P. D., Sartor, A. M., Wang, J., Saurabh, S., Shapiro, L., Moerner, W. E. 2018; 140 (39): 12310–13

    Abstract

    Single-molecule super-resolution fluorescence microscopy conducted in vitrified samples at cryogenic temperatures offers enhanced localization precision due to reduced photobleaching rates, a chemical-free and rapid fixation method, and the potential of correlation with cryogenic electron microscopy. Achieving cryogenic super-resolution microscopy requires the ability to control the sparsity of emissive labels at cryogenic temperatures. Obtaining this control presents a key challenge for the development of this technique. In this work, we identify a red photoactivatable protein, PAmKate, which remains activatable at cryogenic temperatures. We characterize its activation as a function of temperature and find that activation is efficient at cryogenic and room temperatures. We perform cryogenic super-resolution experiments in situ, labeling PopZ, a protein known to assemble into a microdomain at the poles of the model bacterium Caulobacter crescentus. We find improved localization precision at cryogenic temperatures compared to room temperature by a factor of 4, attributable to reduced photobleaching.

    View details for DOI 10.1021/jacs.8b05960

    View details for PubMedID 30222332

  • Direct Selection of Fluorescence-Enhancing RNA Aptamers JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Gotrik, M., Sekhon, G., Saurabh, S., Nakamoto, M., Eisenstein, M., Soh, H. 2018; 140 (10): 3583–91

    Abstract

    RNA aptamers that generate a strong fluorescence signal upon binding a nonfluorescent small-molecule dye offer a powerful means for the selective imaging of individual RNA species. Unfortunately, conventional in vitro discovery methods are not efficient at generating such fluorescence-enhancing aptamers, because they primarily exert selective pressure based on target affinity-a characteristic that correlates poorly with fluorescence enhancement. Thus, only a handful of fluorescence-enhancing aptamers have been reported to date. In this work, we describe a method for converting DNA libraries into "gene-linked RNA aptamer particles" (GRAPs) that each display ∼105 copies of a single RNA sequence alongside the DNA that encodes it. We then screen large libraries of GRAPs in a high-throughput manner using the FACS instrument based directly on their fluorescence-enhancing properties. Using this strategy, we demonstrate the capability to generate fluorescence-enhancing aptamers that produce a variety of different emission wavelengths upon binding the dye of interest.

    View details for DOI 10.1021/jacs.7b10724

    View details for Web of Science ID 000427910700022

    View details for PubMedID 29505267

  • Genetically Targeted Ratiometric and Activated pH Indicator Complexes (TRApHIC) for Receptor Trafficking BIOCHEMISTRY Perkins, L. A., Yan, Q., Schmidt, B. F., Kolodieznyi, D., Saurabh, S., Larsen, M., Watkins, S. C., Kremer, L., Bruchez, M. P. 2018; 57 (5): 861–71

    Abstract

    Fluorescent protein-based pH sensors are useful tools for measuring protein trafficking through pH changes associated with endo- and exocytosis. However, commonly used pH-sensing probes are ubiquitously expressed with their protein of interest throughout the cell, hindering our ability to focus on specific trafficking pools of proteins. We developed a family of excitation ratiometric, activatable pH responsive tandem dyes, consisting of a pH sensitive Cy3 donor linked to a fluorogenic malachite green acceptor. These cell-excluded dyes are targeted and activated upon binding to a genetically expressed fluorogen-activating protein and are suitable for selective labeling of surface proteins for analysis of endocytosis and recycling in live cells using both confocal and superresolution microscopy. Quantitative profiling of the endocytosis and recycling of tagged β2-adrenergic receptor (B2AR) at a single-vesicle level revealed differences among B2AR agonists, consistent with more detailed pharmacological profiling.

    View details for DOI 10.1021/acs.biochem.7b01135

    View details for Web of Science ID 000424723300049

    View details for PubMedID 29283245

  • Super-Resolution Microscopy and Single-Protein Tracking in Live Bacteria Using a Genetically Encoded, Photostable Fluoromodule. Current protocols in cell biology Saurabh, S., Perez, A. M., Comerci, C. J., Shapiro, L., Moerner, W. E. 2017; 75: 4.32.1–4.32.22

    Abstract

    Visualization of dynamic protein structures in live cells is crucial for understanding the mechanisms governing biological processes. Fluorescence microscopy is a sensitive tool for this purpose. In order to image proteins in live bacteria using fluorescence microscopy, one typically genetically fuses the protein of interest to a photostable fluorescent tag. Several labeling schemes are available to accomplish this. Particularly, hybrid tags that combine a fluorescent or fluorogenic dye with a genetically encoded protein (such as enzymatic labels) have been used successfully in multiple cell types. However, their use in bacteria has been limited due to challenges imposed by a complex bacterial cell wall. Here, we describe the use of a genetically encoded photostable fluoromodule that can be targeted to cytosolic and membrane proteins in the Gram negative bacterium Caulobacter crescentus. Additionally, we summarize methods to use this fluoromodule for single protein imaging and super-resolution microscopy using stimulated emission depletion. © 2017 by John Wiley & Sons, Inc.

    View details for DOI 10.1002/cpcb.21

    View details for PubMedID 28627757

  • Removing orientation-induced localization biases in single-molecule microscopy using a broadband metasurface mask NATURE PHOTONICS Backlund, M. P., Arbabi, A., Petrov, P. N., Arbabi, E., Saurabh, S., Faraon, A., Moerner, W. E. 2016; 10 (7): 459-?
  • Fluoromodule-based reporter/probes designed for in vivo fluorescence imaging JOURNAL OF CLINICAL INVESTIGATION Zhang, M., Chakraborty, S. K., Sampath, P., Rojas, J. J., Hou, W., Saurabh, S., Thorne, S. H., Bruchez, M. P., Waggoner, A. S. 2015; 125 (10): 3915–27

    Abstract

    Optical imaging of whole, living animals has proven to be a powerful tool in multiple areas of preclinical research and has allowed noninvasive monitoring of immune responses, tumor and pathogen growth, and treatment responses in longitudinal studies. However, fluorescence-based studies in animals are challenging because tissue absorbs and autofluoresces strongly in the visible light spectrum. These optical properties drive development and use of fluorescent labels that absorb and emit at longer wavelengths. Here, we present a far-red absorbing fluoromodule-based reporter/probe system and show that this system can be used for imaging in living mice. The probe we developed is a fluorogenic dye called SC1 that is dark in solution but highly fluorescent when bound to its cognate reporter, Mars1. The reporter/probe complex, or fluoromodule, produced peak emission near 730 nm. Mars1 was able to bind a variety of structurally similar probes that differ in color and membrane permeability. We demonstrated that a tool kit of multiple probes can be used to label extracellular and intracellular reporter-tagged receptor pools with 2 colors. Imaging studies may benefit from this far-red excited reporter/probe system, which features tight coupling between probe fluorescence and reporter binding and offers the option of using an expandable family of fluorogenic probes with a single reporter gene.

    View details for DOI 10.1172/JCI81086

    View details for Web of Science ID 000362311700021

    View details for PubMedID 26348895

    View details for PubMedCentralID PMC4607111

  • Targeting Dyes for Biology CELL MEMBRANE NANODOMAINS: FROM BIOCHEMISTRY TO NANOSCOPY Saurabh, S., Bruchez, M. P., Cambi, A., Lidke, D. S. 2015: 341–64
  • Near-instant surface-selective fluorogenic protein quantification using sulfonated triarylmethane dyes and fluorogen activating proteins ORGANIC & BIOMOLECULAR CHEMISTRY Yan, Q., Schmidt, B. F., Perkins, L. A., Naganbabu, M., Saurabh, S., Andreko, S. K., Bruchez, M. P. 2015; 13 (7): 2078–86

    Abstract

    Agonist-promoted G-protein coupled receptor (GPCR) endocytosis and recycling plays an important role in many signaling events in the cell. However, the approaches that allow fast and quantitative analysis of such processes still remain limited. Here we report an improved labeling approach based on the genetic fusion of a fluorogen activating protein (FAP) to a GPCR and binding of a sulfonated analog of the malachite green (MG) fluorogen to rapidly and selectively label cell surface receptors. Fluorescence microscopy and flow cytometry demonstrate that this dye does not cross the plasma membrane, binds with high affinity to a dL5** FAP-GPCR fusion construct, activating tagged surface receptors within seconds of addition. The ability to rapidly and selectively label cell surface receptors with a fluorogenic genetically encoded tag allows quantitative imaging and analysis of highly dynamic processes like receptor endocytosis and recycling.

    View details for DOI 10.1039/c4ob02309a

    View details for Web of Science ID 000349401300019

    View details for PubMedID 25520058

    View details for PubMedCentralID PMC4318720

  • Reply to 'Complexity of molecular crowding in cell-free enzymatic reaction networks' NATURE NANOTECHNOLOGY Tan, C., Saurabh, S., Bruchez, M. P., Schwartz, R., LeDuc, P. 2014; 9 (6): 407-+

    View details for DOI 10.1038/nnano.2014.111

    View details for Web of Science ID 000336971600004

    View details for PubMedID 24894472

  • Plasticization of Poly(vinylpyrrolidone) Thin Films under Ambient Humidity: Insight from Single-Molecule Tracer Diffusion Dynamics JOURNAL OF PHYSICAL CHEMISTRY B Bhattacharya, S., Sharma, D., Saurabh, S., De, S., Sain, A., Nandi, A., Chowdhury, A. 2013; 117 (25): 7771–82

    Abstract

    Studies on diffusion dynamics of single molecules (SMs) have been useful in revealing inhomogeneity of polymer thin films near and above the glass-transition temperature (T(g)). However, despite several applications of polymer thin films where exposure to solvent (or vapor) is common, the effect of absorbed solvent molecules on local morphology and rigidity of polymer matrices is yet to be explored in detail. High-T(g) hydrophilic polymers such as poly(vinylpyrrolidone) (PVP) are used as pharmaceutical coatings for drug release in aqueous medium, as they readily absorb moisture, which results in effective lowering of the T(g) and thereby leads to plasticization. The effect of moisture absorption on swelling and softening of PVP thin films was investigated by visualizing the diffusion dynamics of rhodamine 6G (Rh6G) tracer molecules at various ambient relative humidities (RH). Wide-field epifluorescence microscopy, in conjunction with high-resolution SM tracking, was used to monitor the spatiotemporal evolution of individual tracers under varied moisture contents of the matrix. In the absence of atmospheric moisture, Rh6G molecules in dry PVP films are translationally inactive, suggestive of rigid local environments. Under low moisture contents (RH 30-50%), translational mobility remains arrested but rotational motion is augmented, indicating slight swelling of the polymer network which marks the onset of plasticization. The translational mobility of Rh6G was found to be triggered only at a threshold ambient RH, beyond which a large proportion of tracers exhibit extensive diffusion dynamics. Interestingly, SM tracking data at higher moisture contents of the film (RH ≥ 60%) reveal that the distributions of dynamic parameters (such as diffusivity) are remarkably broad, spanning several orders of magnitude. Furthermore, Rh6G molecules display a wide variety of translational motion even at a fixed ambient RH, clearly pointing out the extremely inhomogeneous environment of plasticized PVP network. Intriguingly, it is observed that a majority of tracers undergo anomalous subdiffusion even under high moisture contents of the matrix. Analyses of SM trajectories using velocity autocorrelation function reveal that subdiffusive behaviors of Rh6G are likely to originate from fractional Brownian motion, a signature of tracer dynamics in viscoelastic medium.

    View details for DOI 10.1021/jp401704e

    View details for Web of Science ID 000321236200029

    View details for PubMedID 23777572

  • Evaluation of sCMOS cameras for detection and localization of single Cy5 molecules OPTICS EXPRESS Saurabh, S., Maji, S., Bruchez, M. P. 2012; 20 (7): 7338–49

    Abstract

    The ability to detect single molecules over the electronic noise requires high performance detector systems. Electron Multiplying Charge-Coupled Device (EMCCD) cameras have been employed successfully to image single molecules. Recently, scientific Complementary Metal Oxide Semiconductor (sCMOS) based cameras have been introduced with very low read noise at faster read out rates, smaller pixel sizes and a lower price compared to EMCCD cameras. In this study, we have compared the two technologies using two EMCCD and three sCMOS cameras to detect single Cy5 molecules. Our findings indicate that the sCMOS cameras perform similar to EMCCD cameras for detecting and localizing single Cy5 molecules.

    View details for DOI 10.1364/OE.20.007338

    View details for Web of Science ID 000302138800053

    View details for PubMedID 22453414

    View details for PubMedCentralID PMC3500109