Molecular Imaging of Neurotransmitter Receptor Trafficking In Vivo: Regulation by Neuronal Activity

Jose A. Esteban, Ph.D.

University of Michigan

Funded in June, 2006: $200000 for 3 years
LAY SUMMARY . ABSTRACT . HYPOTHESIS . SELECTED PUBLICATIONS .

LAY SUMMARY

back to top

Exploring the Molecular Basis of Brain Plasticity Through Imaging

The investigators will use molecular imaging in laboratory animals to gain a better understanding of how brain wiring occurs, and continuously changes, through cognitive activities such as learning and memory.     

Neural networks form through brain cell connections (synapses) that occur during development and throughout life as a function of cognition.  Yet scientists know little about how this process of brain plasticity produces modifications in connections (synapses) between brain cells in response to cognitive activities.  One important part of the process is the delivery to synapses of receptors.  The receptors are used by neurotransmitters to pass signals from one cell to another within neural networks. Scientists do not know how physiological brain activity, such as memory, trigger brain cells to add receptors to synapses.

The Michigan researchers hypothesize that sensory stimulation triggers synaptic plasticity by controlling the delivery of neurotransmitter receptors into bran synapses. They will test this hypothesis using two-photon fluorescence imaging in anesthetized laboratory animals.

Significance:  Understanding what triggers the delivery of neurotransmitter receptors into synapses connecting one brain cell to another could improve our understanding of brain plasticity normally and lead to methods to determine how this process may malfunction in people with developmental or mental conditions.

ABSTRACT

back to top

Molecular Imaging of Neurotransmitter Receptor Trafficking In Vivo: Regulation by Neuronal Activity

Synaptic connections in the brain are continuously remodeled in response to neuronal activity. This process, known as synaptic plasticity, is widely accepted as the cellular basis for learning and memory, and it is thought to be altered in several cognitive disorders. An important aspect of synaptic plasticity is the regulated transport of neurotransmitter receptors in and out of synapses. In particular, AMPA-type glutamate receptors (AMPA receptors) can be added to or removed from synapses in an activity-dependent manner, leading to long-lasting changes in synaptic function (long-term potentiation and long-term depression). These forms of synaptic plasticity are widely thought to underlie learning and memory.

An important lag in our understanding of receptor trafficking as an underlying mechanism for synaptic plasticity stems from the fact that receptor delivery at synapses has never been observed in living animals in response to physiological brain activity. In fact, progress in this field is currently hindered by the challenge of visualizing neurotransmitter receptor trafficking in real-time in living brain. This proposal is aimed at directly imaging the synaptic insertion of AMPA receptors in response to neuronal activity in the brain in vivo. Our hypothesis is that physiological sensory stimulation will be able to trigger the mobilization and insertion of AMPA receptors from extrasynaptic dendritic compartments into the synaptic membrane at dendritic spines. To this end we will use two-photon laser scanning fluorescence microscopy combined with expression of GFP-tagged AMPA receptors in the somatosensory cortex of living rats.

Using this combination of molecular biology together with cutting-edge fluorescence microscopy, we will determine (i) the basal dynamics of AMPA receptors at dendritic spines in the intact brain, (ii) the regulated delivery of AMPA receptors at synapses in response to sensory stimulation, and (iii) the signaling cascades that mediate the activity-dependent trafficking of AMPA receptors in vivo.

We believe that this molecular imaging approach in intact living brain is essential for the general goal of understanding how individual molecules contribute to normal brain function and to the pathological alterations associated with mental illness.

HYPOTHESIS

back to top

Hypothesis:
We will test the specific hypothesis that physiological sensory stimulation can trigger synaptic plasticity by controlling neurotransmitter receptor transport into synapses in vivo.

Goals:
Our initial goal is to visualize AMPA-type glutamate receptor transport in cortical neurons in the intact brain. This part of the study will allow us to evaluate the dynamic behavior of AMPA receptors at synaptic terminals in vivo. Subsequently, we will determine whether AMPA receptor trafficking in and out of spines is modulated by physiological sensory stimulation. In particular, we will evaluate whether acute somatosensory stimulation can trigger the mobilization of extrasynaptic AMPA receptors into dendritic spines. And finally, we will interfere with specific intracellular signaling cascades to identify the molecular mechanisms that regulate the synaptic trafficking of AMPA receptors induced by sensory stimulation.

Methods:
We will use lentiviral vectors to drive the expression of GFP-tagged AMPA receptors in pyramidal neurons of the somatosensory cortex (layers 2/3) in living rats. These recombinant receptors will then be visualized in the intact brain of the anesthetized animal using laser-scanning two-photon fluorescence microscopy.

SELECTED PUBLICATIONS

back to top

Gerges N.Z., Backos D.S., Rupasinghe C.N., Spaller M.R., and Esteban J.A. Dual role of the exocyst in AMPA receptor targeting and insertion into the postsynaptic membrane. EMBO J. 2006 Apr 19;25(8):1623-34.

Brown T.C., Tran I.C., Backos D.S., and Esteban J.A. NMDA receptor-dependent activation of the small GTPase Rab5 drives the removal of synaptic AMPA receptors during hippocampal LTD. Neuron. 2005 Jan 6;45(1):81-94.

Gerges N.Z., Brown T.C., Correia S.S., and Esteban J.A. Analysis of Rab protein function in neurotransmitter receptor trafficking at hippocampal synapses. Methods Enzymol. 2005;403:153-66.