Richard H. Kramer

Associate Professor of Neurobiology

Measuring and controlling neural activity in the retina

Neurons in the retina communicate using electrical and chemical signals. We use a combination of optical, electrophysiological, and molecular 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, designed to manipulate or monitor the function of ion channels and synapses.

Current Projects

Optical studies of synaptic transmission in the retina
Rod and cone photoreceptors transmit information to other neurons through specialized structures called ribbon synapses. Insights into how these synapses work has come largely from indirect measurements from postsynaptic bipolar or horizontal neurons in the retina. We are are taking a different approach: we are directly measuring synaptic transmssion from rod presynaptic terminals of rods and cones, using a variety of optical methods. We use lipophilic fluorescence dyes, along with 2-photon, confocal, and electron  microscopy, to track the life cycle of synaptic vesicles in photoreceptor terminals. This cycle starts with vesicles that are "born" by endocytosis of membrane from the plasma membrane. The vesicles become filled with neurotransmitter while they move rapidly within the cytoplasm of the terminal. They then encounter and bind to the synaptic ribbon and ultimately fuse with the plasma membrane by exocytosis, releasing their neurotransmitter to the extracellular space to act on postsynaptic cells. We are examining how illumination regulates this process. We hope to understand how the rate of synaptic vesicle release encodes information about light intensity. In addition to studying individual synapses, optical methods allow us to simultaneously image the behavior of 2-dimensional arrays of synapses while the retina is responding to visual images, providing a vivid "moving picture" of information processing in the retina.

Remote control of neural activity with "light-activated" ion channels
To overcome the loss of retinal photoreceptors in diseases such as retinitis pigmentosa and macular degeneration, scientists are attempting to develop electrical prosthetic devices, in which an implantable array of electrodes is used to stimulate remaining healthy neurons downstream in the visual pathway. We are taking a different approach, which involves direct optical regulation of the electrical activity of neurons without using invasive implantable devices. We have engineered the first ion channel that is directly activated with light. This channels consists of a genetically engineered ion channel protein that is covalently attached to a specially designed light-sensitive molecule, which includes a blocker of the channel's pore. Photoisomerization of the light-sensitive molecule extends or retracts the blocker, opening or closing the flow of ions through the pore. Different wavelengths of light switch the molecule back and forth between blocking and unblocking states, so both channel opening and closing are controllable with light. Expression of these channels in neurons allows their electrical activity to be regulated with flashes of light. Hence illumination with 390 nm light silences action potentials and 500 nm light restores activity. Introduction of light-activated channels into retinal ganglion cell may provide a non-invasive means for rapid, reversible, and spatially accurate manipulation of neural activity, perhaps restoring visual sensitivity to individuals lacking functioning rods and cones.

Selected Publications

Banghart, B., Borges, K., Isacoff, E., Trauner, D., and Kramer, R.H. (2004). Light-activated ion channels for remote control of neuronal firing. Science (submitted).

Rea, R., Li., J., Dharia, A., Levitan, E.S., Sterling, P., and Kramer, R.H. (2004). Streamlined synaptic vesicle cycle in cone photoreceptor terminals. Neuron 41:755-766.

Krajewski, J.L, Luetje, C.W., and Kramer, R.H. (2003). Tyrosine phosphorylation switches off Ca2+/calmodulin inhibition in rod cyclic nucleotide-gated channels. Journal of Neuroscience 23, 10100-10106.

Molokanova, E., Krajewski, JL., Satpaev, D, Luetje, CW, and Kramer, RH (2003).  Subunit contributions to phosphorylation–dependent modulation of rod cyclic nucleotide-gated channels. Journal of Physiology 552: 345-356.

Trivedi, B. and Kramer, R.H. (2002). Patch cramming reveals the mechanism of ling-term suppression of cyclic nucleotides in intact neurons. Journal of Neuroscience 22: 8819-8826.

Savchenko, A., Kraft, T.W., Molokanova, E. and Kramer, R.H. (2001). Growth factors regulate phototransduction in retinal rods by modulating cyclic nucleotide-gated channels through dephosphorylation of a specific tyrosine residue. Proceedings of the National Academy of Sciences, USA 98:5880-5885.

Kramer, R.H. and Molokanova, E.  (2001). Modulation of cyclic nucleotide-gated channels and regulation of phototransduction. Journal of Experimental Biology 204:2921-2931.

Molokanova, E., Savchenko, A., and Kramer, R.H. (2000). Interactions of cyclic nucleotide-gated channel subunits and protein tyrosine kinase probed with genistein. Journal of General Physiology 115: 685-696.

Molokanova, E., Maddox, F., Luetje, C.W., and Kramer, R.H. (1999). Activity-dependent modulation of rod photoreceptor cyclic nucleotide-gated channels mediated by phosphorylation of a specific tyrosine residue. Journal of Neuroscience 19:4786-4795.

Molokanova, E., Savchenko, A., and Kramer, R.H. (1999). Non-catalytic inhibition of cyclic nucleotide-gated channels by tyrosine kinase induced by genistein. Journal of General Physiology113:45-56.

Kramer, R.H. and Karpen, J. (1998). Spanning binding sites on allosteric proteins with polymer-linked ligand dimers. Nature 395: 710-713.

Trivedi, B. and Kramer, R.H. (1998). Real-time patch-cram detection of intracellular cGMP reveals long-term suppression of responses to NO and muscarinic agonists.  Neuron 21:895-906.

Nawy, S., Kramer, R.H., and Walters, R.J. (1998). Regulation of the cGMP-dependent current in On-bipolar cells by calcium/calmodulin-dependent kinase. Visual Neuroscience 15:257-263.

Kramer, R.H. and Karpen, J. (1998). Spanning binding sites on allosteric proteins with polymer-linked ligand dimers. Nature 395: 710-713.

Savchenko, A., Barnes, S.A., and Kramer, R.H. (1997). Cyclic nucleotide-gated channels mediate synaptic modulation by nitric oxide. Nature 390:694-698.

Molokanova, E., Trivedi, B., Savtchenko, A. and Kramer, R.H. (1997). Modulation of rod photoreceptor cyclic nucleotide-gated channels by tyrosine phosphorylation. Journal of Neuroscience 17:9068-9076.

Links

Richard Kramer Lab Home Page

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