Photo of Joel Kralj

Joel Kralj

Assistant Professor
JSCBB room A321
Visit the Kraji Lab website.
Explore Joel Kralj's areas of research and more in Vivo


Postdoc, Harvard University (Adam Cohen)
PhD, Boston University, 2009 (Kenneth Rothschild)


Research Interests: biochemistry, molecular engineering, optics, microfluidics, image processing, membrane voltage sensing, electrophysiology, bacteria, cardiomyocytes, neurons.

Research Narative:

The Kralj lab works to create new tools to study functions of molecules, cells, and organisms. A long-standing goal in biology is to develop a genetically encoded reporter of membrane voltage. Such a reporter would eliminate the requirement for cellular mechanical contact, and greatly expand the scope of potential experiments. Microbial rhodopsins have recently gained interest as sensitive and fast fluorescent voltage indicators, and are being used to rapidly measure electrophysiology from many biological systems, including some that are un-patchable.

Microbial rhodopsins as fast fluorescent voltage indicator

Microbial rhodopsins been used to image electrical transients in rat primary neurons, human iPS-derived neurons and cardiomyocytes, zebrafish, and bacteria. In each instance, investigators collected data that would have been difficult or impossible by patch-clamp or electrode techniques. The discovery of voltage transients in bacteria, in particular, emphasize the importance of membrane voltage as a cellular regulator throughout nature. Imaging offers a chance to study voltage across a variety of organisms and cells, including non-excitable cell types.

Our lab wants to understanding how nature has utilized voltage to control cellular physiology. We develop tools to measure the electrophysiological effects from genome wide libraries. Model systems include E. coli, yeast, and iPS-derived cardiomyocytes. Data collection and analysis combine custom optics, microfluidics, cell and molecular biology, biochemistry, protein engineering, and image processing. Using these techniques, we hope to uncover similarities and novelties of voltage-regulated physiology across kingdoms.

Selected Publications

Hou, JH., Kralj, Joel M., Douglass, AD., Engert, F., Cohen, AE. Simultaneous mapping of membrane voltage and calcium in zebrafish heart in vivo reveals chamber-specific developmental transitions in ionic currents. Frontiers in Physiology (2014)

Hochbaum, DR., Zhao, Y., Farhi, SL., Klapoetke, N., Werley, CA., Kapoor, V., Zou, P., Kralj, Joel M., Maclaurin, D., Smedemark-Margulies, N., Saulnier, JL., Boulting, GL., Sraub, C., Cho, YK., Melkonian, M., Wong, GK., Harrison, DJ., Murthy, VN., Sabatini, BL., Boyden, ES., Campbell, RE., Cohen, AE. All-optical Electrophysiology in Mammalian Neurons Using Engineered Microbial Rhodopsins. Nature Methods (2014)

Kralj, Joel M*., Douglass, AD*., Hochbaum, DR*., Cohen, AE. Optical Recording of Action Potentials in Mammalian Neurons with a Voltage Indicating Protein. Nature Methods, 9(1):90-95. (2012)

Kralj, Joel M., Hochbaum, DR., Douglass AD., Cohen AE. Electrical Spiking in Escherichia Coli Probed with a Fluorescent Voltage Indicating Protein. Science, 333(6040):345-348 (2011)

Bayraktar H, Fields AP, Kralj, Joel M, Spudich JL, Rothschild KJ, Cohen AE. Ultrasensitive Measurements of Microbial Rhodopsin Photocycles Using Photochromic FRET. Photochemistry Photobiology, 88(1):90.

Kralj, Joel M., Amsden, JJ., Spudich, EN., Spudich, JL., Rothschild, KJ. Protein-Chromophore interactions in the blue and green proteorhodopsins. Journal of Physical Chemistry B, 112(37): 11770-76 (2008)

Kralj, Joel M., Bergo, VB., Spudich, EN., Spudich, JL., Rothschild, KJ. The Protonation State of Glu142 differs in the green and blue absorbing variants of proteorhodopsin. Biochemistry. 47 (11): 3447-53 (2008)