John P. Leonard, PhD
Building & Room:
950 S. Halsted St.
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My laboratory is focused on neuronal ion channels. We are interested in both neurotransmitter-sensing and voltage-sensing types of channels. Most recent work involves cell surface receptors for glutamate. Glutamate is the major excitatory neurotransmitter in the mammalian brain. Typically, when glutamate binds to its receptor, a channel forms between the surface and the cytoplasm of the neuron allowing cationic current to flow. We study these ion channels by expressing them in a relatively simple, single-cell system, the Xenopus frog oocyte, that can then synthesize channels and incorporate them into the surface membrane. Because there are no glutamate receptors present in native oocytes, we control the types of channels that will be studied. It is also possible to alter the mRNA encoding a given subunit before injection to alter the channel structure produced. In this way we hope to find particular regions in different glutamate receptors that are responsible for their functional differences.
Our general area of interest is in the mechanisms that synapses have to change strength of transmission as a result of prior activation. For example, we have found that one type of glutamate receptor doubles its activity when re-tested after activation of a different type of glutamate receptor. Among the variety of endogenous agents that can cause modulation of neuronal ion channel we have recently chosen to concentrate on protein kinases. Protein kinases add a phosphate group to certain amino acid residues on the polypeptide chain. Such phosphorylation often modulates function of receptors just as it can for enzymes.
In a particularly robust example of phosphorylation causing neuromodulation, we have found that the activity of one type of glutamate receptor at a glutamatergic synapse could be changed by activity of another type at the same synapse. Selective activation of metabotropic glutamate receptors could change subsequent response at the NMDA subtype of glutamate receptors. This greater than doubling of NMDA receptor current is mediated by activators of protein kinase type C (PKC). NMDA receptors require 2 types of subunits, NR1 and NR2, to bind both glutamate (NR2) and co-agonist glycine (NR1). There are 4 types of NR2 subunits (NR2A-D) found in different NMDA receptors, each with its own influence on functional properties. When the NR2A or NR2B subunit is co-expressed with the NR1 subunit, PKC dramatically increases the current through the receptor. On the other hand, when either the NR2C or NR2D subunit is co-expressed with the glycine binding subunit NR1, it prevents PKC from potentiating the receptor's current flow. Expression of point mutations at potential PKC phosphorylation sites present in NR2A and NR2B but absent in NR2C and NR2D have allowed us to identify 2 serine residues controlling direct action of PKC on NMDA receptors. There are also indirect actions of PKC via tyrosine kinases that phosphorylate different distinct sites on NR2A and NR2B. In collaboration with the Schmidt laboratory in the department, we are constructing and now testing gene-targeted replacement mice containing NR2A or NR2B receptors with these key phosphorylation sites eliminated. These are named Grin2adeltaPKC mice. This will allow us to test the importance of these sites to natural physiology and ultimately, behavior of the whole animal. So far we have found that the Grin2adeltaPKC mice do show behavioral changes, in particular, a decrease in anxiety and less spontaneous alternation although they are as active as normal mice.
(Complete list of publications on Google Scholar)
- Jones, M.L., G-Y Liao, R. Malecki, M. LI, N.M. Salazar, and J.P. Leonard (2012) PI 3-kinase and PKCzeta mediate insulin-induced potentiation of NMDA receptor currents in Xenopus oocytes. Brain Research 1432:7-14.
- Chatterjea, D., E. Hamid, J.P. Leonard, and S. Alford. (2010) Phosphorylation-state-dependent regulation of NMDA receptor short-term plasticity modifies hippocampal dendritic calcium transients. J. Neurophysiology 104: 2203-2213.
- Jones, M.L., and J.P. Leonard. 2005. PKC site mutations reveal differential modulation by insulin of NMDA receptors conatining NR2A or NR2B subunits. J. Neurochem. 92:1431-1438.
- Yang, M., and J. P. Leonard (2001) Identification of mouse NMDA receptor subunit NR2A C- terminal tyrosine sites phosphorylated by coexpression with v-Src. Journal of Neurochemistry 76:1-10.
- Liao, G-Y., D. A. Wagner, M.H.Hsu, and J.P. Leonard. (2001) Evidence for direct protein kinase-C mediated modulation of N-Methyl-D-aspartate receptor current. Accelerated Communication in Molecular Pharmacology 59:1-5.
- Liao, G-Y., M.A. Kreitzer, B.J. Sweetman, and J.P. Leonard. 2000. PSD-95 differentially regulates insulin- and Src-mediated current modulation of mouse NMDA receptors expressed in Xenopus oocytes J. Neurochem. 75: 282-287.
- Logan, S.M., F.E. Rivera, and J.P. Leonard, 1999. Protein kinase C modulation of recombinant NMDA receptor currents; roles for the C-terminal C1 exon and calcium ions. Journal of Neuroscience 19:974-986.
Postdoc Caltech, Pasadena California 1985-1988 Biology
Postdoc: University of Colorado Medical School, Denver Colorado 1982-1985 Physiology
PhD Cornell University, Ithaca New York 1976-1982 Neurobiology and Behavior
BA University of Colorado, Boulder Colorado 1 972-1976 Molecular, Cellular and Developmental Biology