Richard H. Kramer
Professor of Neurobiology*
*And Member, Graduate Group in Biophysics and Vision Science Program
Research Interests
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Nerve cells communicate using electrical and chemical signals. We use a combination of molecular, optical, and electrophysiological methods to study ion channels, the proteins that generate electrical signals, and synaptic transmission, the process that allows a neuron to communicate chemically with other cells. Many of our most recent studies utilize novel chemical reagents to modify the function of ion channels and synapses. This Chemical-Biological approach is designed to allow non-invasive optical sensing and optical manipulation of neuronal activity in intact regions of the nervous system.
Current Projects
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Light-activated ion channels for remote control of neural activity. Neurons have ion channels that are activated directly by voltage, chemicals, mechanical forces, and temperature, but not by light. Using a combination of organic chemistry and molecular biology, we have engineered the first neuronal ion channel that can be directly activated with light. A chemically synthesized "photoswitch" molecule is covalently coupled to a genetically engineered ion channel protein. The photoswitch contains a ligand that binds to the pore of the channel, blocking the flow of ions. When the photoswitch is in its extended (trans) configuration, the blocker can reach the pore and the channel remains closed, but exposure to ultraviolet light triggers photoisomerization to the bent (cis) form of the molecule, retracting the blocker from the pore, and allowing ion flow. The photoswitch can be switched rapidly and repeatedly with different wavelengths of light, allowing precise and consistent control of channel opening and closing. Expression of these channels in neurons allows action potentials to be regulated with flashes of light.
Our first light-activated ion channel has a pore selective for K+ ions ad is therefore inhibitory. However, a variety of light-activated channels can be tailored to have different functional properties that enable control of various aspects of neural activity. The photoswitch molecule can be chemically modified and the target protein can be altered by mutagenesis. Furthermore, an entirely different ion channel protein could be used to generate light-activated channels We currently are pursuing light-activated Na+ channels (the photoswitch is a promising light-activated local anesthetic), light-activated GABA receptors (useful for optical silencing of neurons), and light-activated protein kinases and phosphatases (useful for control of intracellular signaling).
Light-activated ion channels can be used experimentally to manipulate and better understand neural circuits and plasticity. However, light-regulated proteins may also have medical applications. Light-activated channels could serve as "remote control" devices for non-invasive control of neural activity---an alternative to neural prosthetic devices based on implanted electrode arrays. One particularly intriguing target for these channels is in the retina---the one part of the nervous system that is naturally accessible to light. By converting "blind" retinal neurons (e.g. retinal ganglion cells) into artificially photosensitive cells, it may be possible to restore visual sensitivity to blind animals (and perhaps people) that have lost their natural rod and cone photoreceptors to injury or degenerative diseases.
Optical imaging of synaptic transmission in the retina. Rod and cone photoreceptors of the retina transmit information to other neurons through specialized structures called ribbon synapses. Unlike most synapses, ribbon synapses continuously release neurotransmitter, with the sensory stimulus (light) causing a decrease in the rate of release. We are using fluorescent indicator dyes and proteins, along with 2-photon, confocal, and electron microscopy, to track the life cycle of synaptic vesicles in photoreceptor terminals. This process starts with endocytosis, includes interactions with the synaptic ribbon, and ends with exocytosis. The function of the synaptic ribbon is a long-standing mystery and our goal is to directly observe its interaction with synaptic vesicles to understand its role in controlling their release.
We are also examining how physiological stimuli, including light, regulate neurotransmitter release from rods and cones. Because neurotransmitter release occurs through the all-or-none fusion of synaptic vesicles, the graded (“analog”) light responses must be converted into a quantal (“digital”) release event at the photoreceptor synapse. Understanding this “analog-to-digital” conversion is crucial for understanding the encoding of light information at this first synapse in the visual system.
Two-photon imaging of synaptic vesicle release and intracellular Ca2+ is also giving us the first glimpses of other aspects of retinal signal processing, including lateral inhibition and color opponency. The use of activity-dependent dyes allows us to examine not only individual synapses, but also the behavior of 2-dimensional arrays of synapses while we project visual images on the retina. By visualizing many neurons simultaneously, optical recording methods provides a unique opportunity to directly see how retinal information is processed at sequential synaptic layers.
Selected Publications
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Light-induced depolarization of neurons using a modified Shaker K+ channel and a molecular photoswitch. [J.J. Chambers, M. Banghart, D. Trauner, and R.H. Kramer (2006). Journal of Neurophysiology (in press)].
Allosteric control of an ionotropic glutamate receptor with an optical switch. [M. Volgraf, P. Gorostiza, R. Numano, R.H. Kramer, E.Y. Isacoff, and D. Trauner (2006) Nature Chemical Biology 2, 47-52].
Photochemical tools for remote control of ion channels in excitable cells. [R.H. Kramer., J.J. Chambers, and D. Trauner (2005). Nature Chemical Biology 1, 360-365].
Encoding of light intensity at the cone photoreceptor synapse. [S.Y. Choi, R. Rea, B. Borghuis, E.S. Levitan, P. Sterling, and R.H. Kramer (2005). Neuron 48, 555-562].
Imaging light-modulated release of synaptic vesicles in the intact retina: Retinal physiology at the dawn of the post-electrode era. [S.Y. Choi, Z. Sheng, and R.H. Kramer (2005). Vision Research 45, 3487-3495].
Light-activated ion channels for remote control of neuronal firing. [M. Banghart, K. Borges, E. Isacoff., D. Trauner, and R.H. Kramer (2004). Nature Neuroscience 12, 1381-1386].
Streamlined synaptic vesicle cycle in cone photoreceptor terminals. [R. Rea, J. Li, A. Dharia, E.S. Levitan, P. Sterling, and R.H. Kramer (2004). Neuron 41, 755-766].
Subunit contributions to phosphorylation–dependent modulation of rod cyclic nucleotide-gated channels. [E. Molokanova, J.L. Krajewski, D. Satpaev, C.W. Luetje, and R.H. Kramer (2003) Journal of Physiology 552, 345-356].
Tyrosine phosphorylation of rod cyclic nucleotide-gated channels switches off Ca2+/calmodulin inhibition. [J.L. Krajewski, C.W. Luetje, and R.H. Kramer (2003) Journal of Neuroscience 23, 10100-10106].
Patch cramming reveals the mechanism of ling-term suppression of cyclic nucleotides in intact neurons. [B. Trivedi, and R.H. Kramer (2002) Journal of Neuroscience 22, 8819-8826].
Modulation of cyclic nucleotide-gated channels and regulation of phototransduction. [R.H. Kramer and E. Molokanova. (2001) Journal of Experimental Biology 204, 2921-2931].
Mechanism of inhibition of cyclic nucleotide-gated channel by protein tyrosine kinase probed with genistein. [E. Molokanova, and R.H. Kramer, (2001). Journal of General Physiology 117, 219-234].
Growth factors regulate phototransduction in retinal rods by modulating cyclic nucleotide-gated channels through phosphorylation of a specific tyrosine residue. [A. Savchenko, T.W. Kraft, E. Molokanova, and R. H. Kramer (2001) PNAS 98, 5880-5885].
Spanning binding sites on allosteric proteins with polymer-linked ligand dimers. [R. H. Kramer and J. Karpen (1998) Nature 395, 710-713].
Real-time patch-cram detection of intracellular cGMP reveals long-term suppression of responses to NO and muscarinic agonists. [B. Trivedi and R. H. Kramer (1998) Neuron 21, 895-906].
Cyclic nucleotide-gated channels mediate synaptic feedback by nitric oxide. [A. Savchenko, S. A. Barnes, and R. H. Kramer (1997) Nature 390, 694-698].
Last Updated 2006-08-25