Voltage-gated Na+ (Nav) channels initiate the majority of action potentials in the nervous system and alpha subunit mutations are responsible for pathologies ranging from an absence of pain phenotype to epilepsy. A high Nav channel density at the axon initial segment (AIS) and node of Ranvier is believed to regulate action potential threshold within these neuronal compartments. Mechanisms regulating the targeting to these sub- cellular domains, and the functional consequences of such localization, are directly relevant to human health since diseases such as epilepsy are caused by Na+ channel trafficking or localization defects. In addition, loss of appropriate Na+ channel localization to te axon initial segment after ischemic injury likely contributes to neuronal dysfunction and modulation of this relocalization could be a future treatment for stroke. Febrile seizures may originate in the AIS since increased temperature enhances the activity of AIS localized Nav channels. Interestingly, the AIS is structurally remodeled following traumatic brain injury and in models of central nervous system trauma Nav channel blockers are neuro-protective. Despite the physiological and pathological significance of the Nav channels in the AIS, little information exists concerning the trafficking, maintenance, and location-dependent function of these channels. In fact, major debates in the field center around questions such as how Nav channels traffic to the AIS and what percentage of these channels are functional. Research in these areas has been hampered by a lack of fluorescently tagged Nav channel constructs and live cell imaging approaches. This research proposal utilizes novel Nav channel constructs in conjunction with high resolution single molecule imaging approaches to monitor the real-time trafficking, localization, and function of Nav1.6 channels in the soma and AIS of hippocampal neurons.
Specific Aim 1 will test hypotheses relating to the mechanisms of Nav1.6 localization at the axon initial segment.
Specific Aim 2 will examine how Nav1.6 channel activity varies as a function of cell surface location. Hypotheses common to both aims deal with how Nav1.6 responds to ischemia-like neuronal insults. The proposed research will enhance our understanding of the function and regulation of neuronal Nav channels. Preventing the altered Nav channel localization induced by ischemic insult is a potential target for future drug development.
The proposed research studies voltage-gated Na+ channel cell surface localization and function at the single molecule level. Na+ channel dysfunction is responsible for epilepsy, ataxia, cognitive defects, ischemia- induced disease, and could be involved in schizophrenia. Understanding the molecular mechanisms regulating Na+ channel localization and function within specific domains of the neuronal cell surface will enhance the treatment of epilepsy and stoke.