John Marshall

Assistant Professor

Graduate Program in Neuroscience
Ph.D., Cambridge University, England, 1989
(401) 863-2574

In response to hormonal or synaptic stimulation, excitable cells (including smooth muscle, cardiac muscle, and neurons) undergo a diversity of changes in their electrical properties. Changes in channel activity are known to regulate neuronal gene expression, cell death and communication. My lab is studying the regulation of glutamate receptors and potassium channels by protein kinases. Glutamate receptors play key roles in certain forms of synaptic plasticity and the failure to regulate these receptors can cause seizure disorders and neurodegenerative disease. We use patch-clamp techniques to study the electrophysiological properties of cloned glutamate receptors heterologously expressed in mammalian cell lines and Xenopus oocytes to determine how their properties are modulated by kinases such as protein kinase A, CaM-Kinase II and protein-tyrosine kinases (PTK). The PTK receptors, such as EGF, PDGF and the neurotrophin receptors are expressed on fully differentiated neurons and may also play an important role in regulating neuronal excitability. In addition to the glutamate receptor studies we have recently cloned a novel calcium-activated potassium channel that plays an important role in regulating blood pressure. Site-directed mutagenesis methods are used to determine its structure and how the gating properties of this channel are regulated by nitric oxide and other second messengers.

Another focus of the lab is the development of optical methods that utilize the cloned jellyfish green fluorescent protein (GFP). We have engineered a chimeric NMDA/GFP receptor subunit that can be observed by fluorescence microscopy while retaining its normal electrical properties. The tagging of receptors with GFP will enable us to visualize the expression and localization of ion channels in living cells.

Marshall, J., Molloy, R., Moss, G., Howe, J.R. and Hughes, T.E. The Jellyfish Green Fluorescent Protein: A new tool for studying ion channel expression. Neuron 14: 211-215, 1995.

Quattrocki, E.A., Marshall, J. and Kaczmarek, L.K. A shab potassium channel contributes to action potential broadening in peptidergic neurons. Neuron 12: 72-86, 1994.

Marshall, J. and Howe, J.R. (1994) Recombinant Glutamate-Receptor ion channel formed from BluR6 subunits. Biophysical Journal A436.



Unitary properties of heteromeric channels containing the NMDAR1-GFP chimera.
A.Examples of single-channel currents evoked by 1 µM glutamate in an outside-out patch excised from a HEK 293 cell that was co-transfected with the cDNAs encoding teh NMDAR-1GFP chimera nad NMDAR2-A.
B.Histogram of the amplitude of individual sample points obtained from a longer stretch of record from the same patch at 80 mV. The fit to the open sample points gives a mean inward current amplitude of 4.0 pA, which corresponds to a single channel conductance of 50 pS.