Failure of learning and memory is one of the most debilitating aspects of aging and neurodegenerative disease, yet we do not understand the basic mechanisms of these crucial brain processes and we cannot intervene effectively in these deficits. Learning and memory takes place primarily at synapses. Presynaptic calcium (Cav2.1) channels initiate neurotransmitter release at most synapses in the brain. The activity of these channels is tightly regulated by a large complex of signaling proteins, including calmodulin and related calcium sensor proteins. The classic work of Katz and Miledi in the 1960's first described short-term synaptic facilitation and depression. These forms of short-term synaptic plasticity shape the postsynaptic response to trains of action potentials impinging on the presynaptic terminal and thereby encode information contained in the frequency and pattern of action potentials for transmission to the postsynaptic cell. The mechanisms that underlie short-term synaptic plasticity on the presynaptic side of the synapse remain poorly understood. Our recent work has implicated Ca channel regulation as an important component of short-term synaptic plasticity. Studies of Cav2.1 channels transfected in individual superior cervical ganglion neurons in cell culture showed that both short-term synaptic facilitation and the rapid phase of synaptic depression are blocked by mutations that prevent facilitation and inactivation of Cav2.1 channel activity by calcium/calmodulin and other calcium sensor proteins. Based on these results, we hypothesize that regulation of Cav2.1 channels by calmodulin and calcium sensor proteins is an important contributor to short-term synaptic plasticity at synapses in the hippocampus and that this form of synaptic plasticity is important for spatial learning and memory. We will address this hypothesis at the molecular level by developing a high-resolution molecular model for the interacting domains of Cav2.1 channels and CaS proteins based on Rosetta structural modeling, chemical crosslinking, and high-resolution mass spectrometry. We will address this hypothesis at the functional and behavioral levels using a recently developed knock-in mouse line in which the IQ-like motif that is required for CaM-dependent facilitation of Cav2.1 channels has been mutated to prevent facilitation of channel activity (Cav2.1/IM-AA mice). We will determine the role of regulation of Cav2.1 channels in short-term synaptic plasticity of neural circuits in hippocampal slices from wild-type and IM-AA mutant mice, which are deficient in presynaptic plasticity in the nerve terminals of CA3 neurons. We will explore the role of regulation of Cav2.1 channels and short-term synaptic plasticity in spatial learning and memory in wild-type and IM-AA mutant mice, which are deficient in context-dependent fear conditioning. Our experiments with this unique mouse model will give new insights into the mechanism of short-term presynaptic plasticity in hippocampal neurons and its role in spatial learning and memory. This information will be essential to understanding of failure of spatial learning and memory in aging and disease.
Learning and memory depend on modification of the strength of communication between nerve cells at synapses, a process called synaptic plasticity. In this work we will analyze the molecular and cellular mechanism of short-term synaptic plasticity, which takes place on the millisecond time scale and is important for encoding and transmitting information in neurons. Our results will help to understand learning and memory in the normal brain and pave the way for future understanding of the failure of these processes in aging and neurodegenerative disease.
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