Martha Constantine-Paton

Our work concentrates on the glutamatergic and GABAergic neurotransmitter systems in the midbrain optic lobes, (the superior colliculus of rodents and the optic tectum of amphibians). The normal development of the visual projections to these regions is well understood and they are readily accessible for assays and manipulations of synaptic and structural plasticity that would be difficult or impossible in other regions of the central nervous system.


Currents

These studies use whole-cell patch clamping techniques in conjunction with molecular and anatomical analyses to examine the relationship between structural and functional changes in synaptic currents and the molecular composition and post-translational modifications of these neurotransmitter receptors. Much of this work has concentrated on the the N-methyl-D-aspartate subtype of glutamate receptor (NMDAR) because it is the most prominent ion-passing glutamate receptor on young CNS neurons, it has the unique attributes of requiring both glutamate binding and membrane depolarization to activate it and because, when the NMDA receptor is activated, it passes significant quantities of the important second messenger Ca++ into neurons. Our recent work in this area has demonstrated two levels of control of NMDAR synaptic currents: a slow decrease in receptor function resulting from the addition of a mature NMDAR receptor subunit to the molecular complex at the synapse and a rapid post-translational modification of NMDAR function resulting from increases in the activity of the Ca++ calmodulin dependent phosphatase calcineurin.

Activity-Dependent Regulation of Glutamate Receptor Function

Mechanisms that can titrate glutamate receptor function as the brain matures are important because excessive glutamatergic activity produces seizures and cell death while too little glutamate receptor function impedes normal synaptic and circuit development. In addition, glutamate receptor down-regulation which occurs as a normal correlate of maturation is frequently associated with a marked decrease in structural plasticity. Thus a cellular and molecular understanding of receptor down-regulation should suggest approaches to upregulating plasticity in select regions of mature brain to facilitate recovery from trauma-induced dysfunction or from genetic or environmentally induced abnormalities in early brain activity. Our studies are focused on normal mice and rats, and mice with mutations in molecules necessary for glutamate neurotransmission. Recently we have found that glutamatergic and GABAergic synaptic development are quite tightly coupled and balanced. We have also shown that abnormally low levels of NMDAR receptor function retards glutamatergic synaptic development, whereas complete blockade has only minor effects.

Signaling Pathways Downstream of NMDAR Activation During Development

These studies aim to dissect the events downstream of NMDAR function at young synapses in order to understand how activation of this receptor is linked to dynamic changes in young neuronal contacts. These changes include filopodial motility, neurite elongation, “compensatory” sprouting of a remaining projection when a converging input is removed and the stabilization of inputs that are most effective in activating NMDAR currents. Some of these studies utilize primary Xenopus or rodent visual neurons in tissue culture in conjunction with studies that alter their activity, examine morphological changes over time and meaure Ca++ influx. We are also currently using a GFP transgenic mouse in which, via adenovirus transfection, RFP labeled synaptic vesicles allow visualization of changes in a small population of presynaptic processes adjacent to GFP labeled dendrites whose activity can be controlled by voltage-clamping.

Another set of studies are aimed at identifying the specific molecular changes at synapses that are immediately downstream of NMDAR activity in young neurons. For example, we have found that controlled eye-opening in rat pups trigger a major trafficking of PSD-95 to visual synapses within 6 hours of eye-opening. This biochemical change is associated with the rapid appearance of new NMDAR currents followed by changes in AMPAR currents typical of LTP.

Finally, we have begun to explore the regulation of glutamate receptors in neurological disease. Thus, with collaboration at Massachusetts General Hospital, we have begun to examine glutamate receptors on normal motorneurons and in transgenic mice carrying a mutant superoxide dismutase gene that has been linked to familiar amyotrophic lateral sclerosis (ALS).


Yoshii, A., Sheng, M.H., and Constantine-Paton, M. Eye-opening induces a rapid dendritic localization of PSD-95 in central visual neurons. PNAS 100:1334-1339 (2003).

Townsend, M., Yoshii, A., Mishina, M., and Constantine-Paton, M. Developmental loss of miniature N-methyl-D-aspartate receptor currents in NR2A KO mice. PNAS 100:1340-1345 (2003).

Colonnese, M.T., Shi, J. and Constantine-Paton, M. Chronic NMDA recoptor blockade from birth delays the maturation of NMDA currents but does not affect AMPA/KA currents. J. Neurophysiol. 89:57-68 (2003).

Miskevich, F., Lu, W., Lin, S-Y., Constantine-Paton, M. Interaction between metabotropic and NMDA subtypes of glutamate receptors in sprout suppression at young synapses. J. Neurosci. 22(1):226-228 (2002).

Shi, J., Townsend, M. and M. Constantine-Paton Activity-dependent induction of tonic calcineurin activity mediates an abrupt developmental down-regulation of NMDA receptor currents. Neuron, 28:103-114 (2000).