How does a memory outlast the lifetime of the molecule that encodes it? More than two decades ago, Francis Crick had the foresight to speculate that perhaps a multimeric protein could serve as a molecular memory by sharing its activation state with newly synthesized proteins through subunit exchange in order to store a memory for years. Ca2+-calmodulin dependent protein kinase II (CaMKII) was identified as an enzyme that may fit this description. For example, mutation of CaMKII at sites critical for its function results in severe learning and memory defects. CaMKII is activated at a threshold neuronal spike frequency and is crucial to long-term potentiation (LTP). A major obstacle to understanding LTP is the absence of understanding how regulatory pathways recruited during this initial high-frequency stimulus are able to remain persistently active in the face of ongoing protein turnover. Our recent work has shown that CaMKII exchanges subunits between holoenzymes in an activation-dependent manner. Importantly, kinase activity is conferred to unactivated CaMKII holoenzymes by trans-phosphorylation as a consequence of subunit exchange, thereby potentiating the activation signal past the time of protein degradation. This cycle may continue indefinitely. Our work is aimed to further investigate this phenomenon, specifically in respect to its role in LTP. Our major research goals are to: 1) understand the role of the unique biophysical properties of CaMKII (how linker length affects activation, frequency dependence and subunit exchange) that contribute to its potential for being a `memory molecule,' and 2) investigate the properties of CaMKII (such as subunit exchange and changes in gene expression) in cellular systems to determine its physiological role in LTP. These challenging goals require the synthesis of information obtained from the molecular level (protein structure and regulation) to the cellular level (mammalian cell culture) and finally to the animal level (transgenic mice), which will be for future study. Completion of the proposed work will allow us to better address neurologic disease progression as it affects memory, such as pathologies seen in Alzheimer's, dementia, and traumatic brain injury.
A memory can last for decades, but the molecular components of our cells are degraded on the order of minutes to days. We are fascinated by this significant discrepancy and will interrogate this using a research program centered around a molecule which is known to play a crucial role in long term potentiation: Ca2+- calmodulin dependent protein kinase II. We plan to investigate this important problem at the molecular and cellular levels.
