Study of synaptic plasticity is not only important for understanding normal brain function such as learning and memory but also is necessary for identifying the underlying causes of many neurological and mental diseases. A tractable model system for investigating synaptic plasticity is long-term presynaptic facilitation of sensory-to-motor neuron synapses in Aplysia, an elementary form of learning and memory. Long-term facilitation requires signal transduction from the neurotransmitter 5-HT to the nucleus for activation of gene transcription by the cAMP- responsive element binding protein (CREB). All along the signaling pathway inhibitors operate to prevent memory formation in the absence of adequate stimuli. These inhibitors are degraded by the ubiquitin-proteasome pathway. Our overall goal is to determine how the ubiquitin-proteasome pathway functions to remove proteins that inhibit long-term facilitation and to understand the important regulatory checkpoints in this pathway. An earlier discovery showed that the regulatory subunit of the cAMP-dependent protein kinase is degraded by the ubiquitin pathway to make the kinase persistently active. Our hypothesis is that modulation of the regulatory complexes attached to the enzymatic core of the proteasome contributes to the increased degradation of R subunits and other substrates. Our recent preliminary data indicate that two subunits of the regulatory part of the proteasome play a critical role in the formation of long-term facilitation. We also have direct evidence for removal of CREB 1b, a CREB repressor, by the ubiquitin-proteasome pathway. The proteolysis of CREB 1b occurs during a time window preceding CREB activation. Our results indicate that the activity of the enzyme that targets CREB lb for proteolytic removal is regulated by phosphorylation.
Our first aim i s to show that the up-regulation of two proteasome regulatory subunits plays an important role in induction of long-term facilitation.
Our second aim i s to isolate and study the enzyme that targets CREB 1b for degradation and to demonstrate that it plays a critical role in inducing long-term facilitation. The concept that the ubiquitin-proteasome pathway operates to determine the threshold for gene induction by CREB and consequently threshold for formation of long-term memory could be widely applicable to understanding other forms of neuronal plasticity. Our approach that uses a combination of molecular, cellular and electrophysiological techniques would enable us to gain novel insights into the mechanisms underlying synaptic plasticity. Since alterations in CREB activation as well as the ubiquitin- proteasome pathway are seen in several abnormalities of the brain, these insights would be useful for identifying the causes of neuronal dysfunction as well as designing therapeutic interventions.
