We all have memories that date back to our youth;we remember the house we lived in at age 4;we remember a favorite schoolteacher. The mechanism for storing these memories is believed to be the long-term plasticity of synaptic connections within specific neuronal circuits. However, this putative cellular basis of memory relies on proteins that typically have lifetimes far shorter than the memory. Here exactly lies a fundamental problem of long-term memory and synaptic plasticity: How can memories be stored for a human lifetime on the basis of proteins that are continuously degrading? Recently, it was shown that the brain-specific PKC isoform, protein kinase M??(PKM?), plays a unique role in maintaining both late long-term potentiation (L-LTP) of synapses and long-term memory. This crucial observation, however, does not explain how PKM??can overcome the natural degrading effect of protein turnover and diffusion. The central hypothesis of this proposal is that PKM??, through its control of its own synthesis, can form a bi-stable system, which can account for the maintenance of synapse specific long-term plasticity and memory. Here we propose to mathematically formulate this hypothesis within a biophysical model, and to analyze this model so as to propose testable experimental predictions. We then will directly test these predictions on PKM?-mediated persistent synaptic potentiation, using novel techniques tailored for testing the theory. Intellectual Merit: The finding that PKM??is both necessary and sufficient for the maintenance of synaptic plasticity and long-term memory has fundamentally changed the field of learning and memory, but much needs to be learned about the mechanisms that can actually accomplish the persistence of long-term plasticity and memory. This proposal addresses these questions using a combined theoretical and experimental approach. Such a theory in which bi-stability depends on regulation of translation is novel not only for neuroscience but also for biology in general. Our collaboration is uniquely qualified to carry out the proposed work because the Shouval lab has ample experience in modeling synaptic plasticity in collaboration with experimental groups, and the Sacktor lab has pioneered the science of PKM??and has ample experience with the proposed techniques. The experimental techniques include two new methodologies necessary for testing the predictions. First, we propose to test the model's predictions on protein translation in L-LTP, not by general protein synthesis inhibitors that may have issues of toxicity and indirect effects, but by use of antisense oligodeoxynucleotides directed to the translation start site of PKM??mRNA to specifically block PKM??synthesis in induction and maintenance. Second, because PKM?-mediated potentiation is both highly stable and yet rapidly reversible, we will use a fast-flow hippocampal slice chamber optimized for the study of the maintenance of L-LTP to test key predictions of the model. The proposed stochastic simulations of translation-dependent bi-stability are also novel in computational biology. Broader Impact: As the first demonstrated molecular mechanism of experience-dependent, long-term information storage in the brain, PKM??has significant clinical implications, and within the last year has been shown to contribute to in the biology of a variety of neurological and psychiatric diseases, including post-traumatic stress disorder, central neuropathic pain, and drug abuse. In order to assist the rapidly growing interest in PKM??in many labs, we will make our model accessible to the larger community, allowing for other scientists to test, modify, and incorporate their findings into the model, thus accelerating the pace of scientific discovery. Because an important goal for NSF is to integrate research and education, we will train a diverse pool of students. Our labs already train undergraduates, the Shouval lab takes undergraduates each summer through an REU program (PI S. Cox, Rice), and a UT system grant (PI H. Shouval), and local undergraduates throughout the year, and the Sacktor lab has had a long history of mentoring local disadvantaged high school students (e.g., through the Intel program). Both labs are dedicated to public outreach;for example, an article on PKM??and memory was on the front page of The New York Times. We are eager to extend this type of outreach to the domain of the interaction between theory and experiment in biological sciences.
Recently, the persistent action of PKMzeta has been implicated in the maintenance of drug seeking behavior, with inhibition of PKMzeta in brain circuits underlying addiction abolishing drug-reward memory (e.g., Li et al. J Neurosci 31:5436 ). Therefore, understanding the mechanisms mediating the persistence of PKM^ function may be essential for understanding the persistence of addiction.
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