Microbes and Pathogenesis

How do microorganisms interact with their environment and sense mechanical cues? How is cell division machinery organized and how does it sense force? How do forces get transmitted to the nucleus and change gene? These are all questions that we are working towards answering, through the use of model systems of bacteria, fungi and archae.

Growing E.coli, as captured by Jeesun Lim of the Kenney lab. Specific microscope and magnification used: AP Delta Vision (100X) Imaging method: GFP-plasmid (E.coli)

Regulating Expression

Mechanotransduction plays a critical role in inducing virulence gene expression in microbes, enabling them to survive in a host. The two-component EnvZ/OmpR system is required for virulence in most pathogens. The osmosensor EnvZ acts through OmpR to detect and transmit osmotic stress to the nucleus and thus regulate gene expression. Using single molecule approaches and atomic force microscopy, we are able to examine the behavior of these regulators.

The importance of mechanical forces on the regulation of gene expression is also highlighted in our studies on bacterial genome organization. Exploring this theme in conjunction with how proteins sense external forces has revealed the significant influence of mechanical factors on transcriptional control. Utilizing highly sensitive single molecule manipulation and imaging techniques we are able to measure bi-molecular interactions, providing novel insights into how the detection of external forces involves conformational changes in proteins.  The high resolution detection of such conformational changes at a single molecule level is key to understanding the mechanism of bacterial sensing.

Super-resolution microscopy showing a reconstructed image of the actin cytoskeleton (red) of a mouse embryonic fibroblast. Image captured by Hu Shiqiong of the Hersen lab.

Dividing Forces

The mechanisms regulating assembly and disassembly of cell division machinery and force generation by this machinery are not wholly understood. Our focus is to unravel the actin-independent cytokinetic mechanisms in non-eukaryotic organisms, such as fungi, bacteria and archae. Studies of eukaryotes have greatly advanced our understanding of the bacterial cytoskeleton. Studies into bacterial and archaeal cytokinesis might therefore similarly provide new insight into eukaryotic cytokinesis and cytoskeletal organization. To this end we combine multiple approaches, including genetics, live cell imaging and physical modeling.

N-ethylmaleimide-myosin coated beads linked to actin filaments (green) as captured by Yuan Xin of the Bershadsky and Jie labs.

Nuclear Communications

External cues are passed on to the nucleus through signaling cascades, which result in the transcription of a specific set of genes that allow the cell to adapt to its environment. Although many signaling pathways are well described in the literature, there are only a few studies quantifying their time scales and capacity to convey information – both of which are crucial for timely cellular adaptation to be executed. We look to advanced micro-fabrication and microfluidic experiments to help us explore the frequency response of single cells when subjected to a periodically changing environment.

Selected Publications

Srinivasan, R, M Mishra, FY Leong, KH Chiam and MK Balasubramanian. The Bacillus anthracis tubulin-related protein TubZ assembles force-generating polymers in the absence of other bacterial proteins. Cytoskeleton 68:501-511(2011).

Chen, H, X.Y. Zhu, P.W. Cong, M.P. Sheetz, F. Nakamura, and J. Yan. Differential Mechanical Stability of Filamin A Rod Segments. Biophysical Journal 101:1231-1237(2011).

Fu, H.X., B.S. Freedman, C.T. Lim, R. Heald, and J. Yan . Atomic Force Microscope Imaging of Chromatin Assembled in Xenopus laevis Egg Extract. Chromosoma 120:245-254 (2011).

Chen, H, H.X. Fu, X.Y. Zhu, P.W. Cong, F. Nakamura, and J. Yan. Improved High Force Magnetic Tweezers for Stretching and Refolding of Proteins and Short DNA. Biophysical Journal 100: 517-523(2011)

Fu, H.X., H Chen, X.H. Zhang, Y QU, J.F. Marko and J. Yan . Transition dynamics and selection of the distinct S-DNA and strand unpeeling modes of double helix overstretching. Nucleic Acids Research 39:3473-3481(2011).

Walthers, D., Y Li, Y Liu, S A Ganesh, J Yan and L.J. Kenney (2011). The salmonella enterica response regulator SsrB relieves H-NS silencing by displacing H-NS bound in polymerization mode and directly activates transcription. Journal of Biological Chemistry 286: 1895-1902.

le Digabel J, Biais N, Fresnais J, Berret JF, Hersen P, Ladoux B (2011). Magnetic micropillars as a tool to govern substrate deformations. Lab Chip. 11(15):2630-6.

Miermont A, Uhlendorf J, McClean M, Hersen P. (2011). The Dynamical Systems Properties of the HOG Signaling Cascade. J Signal Transduct. 2011: 930940.