Excitability, synaptic transmission, and microcircuits

High-resolution voltage-imaging tools provide a unique window into brain function. By resolving subthreshold events in vivo, voltage imaging can probe the synaptic inputs to a neural circuit. In combination with optogenetic activation or silencing, voltage imaging can resolve the separate roles of synaptic excitation and inhibition. We are using these tools for detailed studies of cortical and hippocampal microcircuits in mice, and of spinal microcircuits in zebrafish.

In vitro voltage imaging provides high-throughput measures of intrinsic excitability of neurons in culture or in acute brain slices, and of synaptic transmission between genetically defined neuronal sub-types. These assays can probe in detail the physiological roles of genetic or pharmacological perturbations and are a powerful tool for disease modeling and drug discovery.

Studies in vivo

Studies in vitro

Voltage imaging tool development

Microbial rhodopsin proteins are found in aquatic organisms throughout the world and in all kingdoms of life. In the wild, these proteins transduce sunlight into changes in membrane potential, which their host organisms use for phototaxis and solar energy capture. We engineered microbial rhodopsins to run in reverse: to transduce changes in membrane potential into an optical signal. These voltage-indicating proteins provide a fast and sensitive tool for monitoring electrical dynamics in cells.

To be used effectively, reporter proteins must be imaged via high-speed and high-sensitivity microscopes. The experiments produce torrents of data, which must be stored and analyzed. Our technology development efforts span from protein engineering to software development.

Opsin spectroscopy and engineering

Microscopes and algorithms

Voltage imaging throughout life

Every membrane-enclosed structure can, in principle, support a voltage difference between the inside and outside. We have developed tools to combine optical perturbation and imaging of voltage in systems across scales of biological organization, including:

  • Bacteria
  • Human stem-cell-derived cardiomyocytes
  • Engineered excitable cell lines
  • Zebrafish heart in vivo

Many of the approximations and intuitions developed for neuroscience do not apply in the context of systems with vastly different sizes, ionic compositions, or charge carriers. We are interested in exploring bioelectrical signaling in unconventional regimes.


Synthetic electrophysiology and bioelectrical patterning

The Hodgkin-Huxley equations of electrophysiology have the same mathematical structure as the Turing reaction-diffusion equation which is often used to describe biological pattern formation during embryonic development. We are studying the interactions of geometry, topology and electrophysiology. How does the geometry of a tissue affect its excitability properties? Can bioelectrical systems spontaneously break spatial symmetry to form steady-state patterns of membrane voltage? What roles does bioelectrical signaling play in embryonic development?

These studies combine cell lines engineered to express defined sets of ion channels and physiological experiments in developing embryos.


Membrane and DNA mechanics

The mechanical properties of cellular materials modulate their biochemical function, but mechanical forces are difficult to measure in situ. The lipid bilayer plasma membrane is often thought of as a two-dimensional fluid. We found that in mammalian cells, the presence of transmembrane proteins tethered to the underlying cytoskeleton induces a transition to a gel-like state. As a result, membrane tension can be highly heterogeneous within individual cells. We are studying the mechanics and biology of local changes in membrane tension, and how cells regulate the propagation of these changes.

The mechanical properties of DNA are also important for governing its function. Double-stranded DNA is stiff, but single-stranded DNA is flexible. Under high curvature, double-stranded DNA can melt to become locally single-stranded. We studied the interaction of DNA bending, melting, and underlying sequence.

Membrane mechanics

DNA mechanics

Past research


tel: (617) 496-9466
email: cohen@chemistry.harvard.edu
Office: Mallinckrodt 115


Harvard University
12 Oxford Street
Cambridge, MA 02138

Administrator: Tracey Schaal

tel: (617) 496-8233
email: schaal@chemistry.harvard.edu