MOLECULAR BIOLOGICAL APPROACH TO LTP IN THE HIPPOCAMPUS

A critical feature of the dementia of the Alzheimer type is an interference with the formation of both short- and long-term memory. One clue that has emerged from a family of recent studies is that the hippocampus is involved in human memory and that damage to only the CA1 region is sufficient to impair the normal conversion of short- to long-term memory. Since LTP occurs in the CA1 region, it now becomes possible to begin to explore in cellular and molecular terms several elementary questions pertaining to normal memory storage. We have been exploring the mechanisms underlying memory in a simple invertebrate system, the monosynaptic component of the gill- and siphon-withdrawal reflex in Aplysia. We found that the proteins synthesized for long-term memory are utilized for two molecular mechanisms: 1) A transcriptionally-dependent persistence in the phosphorylation of the same substrate proteins phosphorylated in the short-term. This results from transcriptionally-dependent depression, perhaps by proteolytic cleavage, of the level of the regulatory subunit of the cAMP-dependent kinase, with the result that the catalytic subunit becomes constitutively active, in the absence of an elevated level of cyclic AMP. This persistence in kinase activity gives the long-term process its striking resemblance to the short-term process. 2) The activation of a growth process whereby new synaptic terminal are formed. This growth process is correlated with, and perhaps results from, the activated in mammalian cells in response to growth factors. The program outline in this proposal attempts to extend to the mammalian brain, and specifically to hippocampal LTP in the CA1 region the approach we have developed in our work on Aplysia. We now plan to explore in the hippocampus six interrelated questions: 1) What is the pattern of phosphorylation produced by LTP? How does the pattern established during the maintenance phase of LTP (2-3 hrs), compare to that of the induction phase (30-60 min)? 2) How does the pattern of phosphorylation, mediated by the C kinase, the Ca2+/calmodulin- dependent kinase or other known second messenger kinases compare to the pattern during initiation of LTP? 3) Does the maintenance of LTP involve persistent phosphorylation by one of these kinases? 4) Is this phosphorylation in the maintenance phase induced by second messengers, transcriptionally-dependent? 5) Does LTP depend on the synthesis of new proteins and mRNAs? 6) If so, what are the genes and proteins whose expression is changed following LTP?