News and Publications

Organization and Function of the Neuronal Cytoskeleton

S. Halpain, S. Graber, A. Hipolito, R. Lim, R. Ozer

Throughout the life span of an organism, neuronal structure and synaptic stability are regulated by transmembrane signals, including synaptic activity itself. Morphologic complexity of neuronal form is essential to the brain's ability to store and process signals. With age, the physiologic strength of synapses tends to weaken, and the number of synapses decline. We are investigating the molecular mechanisms that regulate the structure and function of neurons during development, during normal aging, and in experimental models of stroke.

We initially focused on the neuron-specific microtubule-associated protein MAP2 as a target of the neurotransmitter glutamate, the major excitatory neurotransmitter in mammalian brains. Among its other functions, MAP2 is an important regulator of the stability of microtubules in neuronal dendrites. Using biochemical approaches, we found that MAP2 regulates the dynamic properties of microtubules and has a pronounced stabilizing effect on individual microtubules in vitro. This stabilizing effect is altered by phosphorylation. The phosphorylation state of MAP2 is itself dynamically regulated in response to activation of glutamate receptors. We discovered that calcineurin is an important regulator of MAP2, and additional studies indicated that calcineurin is enriched in postsynaptic dendritic spines and regulates the stability of actin filaments.

We recently studied MAP2 in an elegant in vivo model of synaptic plasticity: the rat olfactory system. We found that phosphorylation of MAP2 is regulated by synaptic input from a chemosensory tissue, the nasal epithelium. Long-term blockade of olfactory stimulation resulted in a time-dependent decrease in phosphorylation of MAP2 at serine-136 in the CNS target tissue, the olfactory bulb.

This study provides a rare link in a living vertebrate animal between neural activity and changes in a specific phosphoprotein target. Biochemical experiments are under way to define the function of phosphorylation of MAP2 at this and other phosphorylation sites. Molecular biological and protein biochemistry approaches have suggested additional functions for MAP2, beyond microtubule stabilization.

Investigations into calcium-dependent signal transduction pathways indicated that L-type voltage-sensitive calcium channels are a crucial player in the weakening of synaptic function in the aging brain. During normal aging, L-channels influence synaptic plasticity by 2 different mechanisms, both of which enhance a type of synaptic weakening known as long-term depression and inhibit the induction of synaptic enhancement known as long-term potentiation. These findings suggest potentially useful therapeutic targets in neuronal aging and new hypotheses on molecular mechanisms at the synapse.

PUBLICATIONS

Norris, C.M., Halpain, S., Foster, T.C. Reversal of age-related alterations in synaptic plasticity by blockade of L-type Ca²⁺ channels. J. Neurosci. 18:3171, 1998.

Philpot, B.D., Lim, J.H., Halpain, S., Brunjes, P.C. Experience-dependent modifications in MAP2 phosphorylation in rat olfactory bulb. J. Neurosci. 17:9596, 1997.