This is a request for a Senior Scientist Award. The importance of studying synaptic function at the molecular level is most obvious for understanding mental and neurological diseases where psychopharmacology and modern molecular genetics suggest an underlying synaptic malady. Long-term presynaptic facilitation (LTF) of sensory-to-motor synapses, which is a form of plasticity underlying behavioral sensitization in the marine mollusk Aplysia, is a convenient experimental model for understanding the molecular events that underlie memory storage. Two major protein kinases, cAMP-dependant protein kinase (PKA) and protein kinase C (PKC), operate to bring about LTF. These enzymes function in the complex mechanisms that bridge the induction and consolidation phases of memory, first by promoting new gene expression needed for the ultimate consolidation, and second by maintaining the stimulated neuron in an interim facilitated state. Thus, during the development of LTF, persistant protein phosphorylation results in the enhancement of synaptic strength by increasing the output of neurotransmitter at existing synapses; later, the memory is consolidated by normal output of transmitter at an increased number of new synapses. Both PKA and PKC induce new protein synthesis through phosphorylations that lead to the activation of transcription factors, including ApC/EBP (the homolog of vertebrate C/EBPbeta. We now aim to characterize how PKC phosphorylation regulates the ongoing cascade of new protein synthesis that is the molecular basis of memory storage in LTF. We find that the ubiquitin-mediated degradation of ApC/EBP is controlled by PKC protein phosphorylation(s), and that the regulatory (R) subunits of PKA decrease in sensory neurons due to ubiquitin-mediated proteolysis. A decreased R/C ratio produces a kinase more sensitive to subsaturating cAMP and sets the baseline extent of protein phosphorylation within the neuron at a higher level for at least 24 h; this change could be the molecular mechanism underlying an intermediary form of memory. Our working idea is that signal transduction by facilitating transmitter (e.g., serotonin) activates PKA, which then triggers a molecular cascade in the nucleus involving cAMP-reactive elements for transcription activator proteins and effector proteins, one or more of which alter the ubiquitin-proteasome in sensory neurons.
