Target derived neurotrophins regulate neuronal survival and function. Little is known about the mechanisms by which neurotrophins regulate events in the nerve cell body when presented to nerve ending that may be as far as a meter away. Previously, we have shown that neurotrophin receptors (Trks) themselves serve as rapid retrograde signal carriers in rat sciatic nerve axons. The research described here builds upon this work. We have three specific aims:
Aim one is to learn how Trks function as retrograde signal carriers. Our preliminary data suggest that a specialized vesicular transport process is utilized to convey Trk signal generating particles in the retrograde direction. To test this hypothesis, activation state-specific antibodies will be used as immuno-electron microscopy reagents and as immunoaffinity reagents to localize and isolate the Trk signal carrying complex in rat sciatic nerve axons. Temperature sensitive mutations of dynamin will be expressed in compartmented cultures of DRG neurons to evaluate the role of receptor internalization in retrograde signaling. Retrograde movement of activated Trks will be visualized in living DRG neurons using Green Fluorescent Protein tags.
Aim two is to learn how P-Trk signal arriving at the nerve cell body is converted to a nuclear signal. Our preliminary data suggest that Trk signal arriving from a remote source of stimulation (the nerve ending) is processed by ERK isoforms distinct from the ones that process Trk signal from a local source (the cell body). We will use genetic and biochemical approaches to identify ERKs that are activated by retrograde signal and evaluate their role(s) in nuclear responses.
Aim three opens a new avenue of inquiry. Our preliminary data suggest that PI3 kinase activity is required for nuclear responses to remote, but not local, neurotrophin stimulation. PI3 kinases are known to regulate membrane trafficking. In addition, phospholipid products of PI3 kinases serve as activators for serine/threonine protein kinases such as Akt. Genetic and biochemical approaches will be used to test the hypothesis that one or both of these PI3 kinase functions are required for retrograde signaling through long axons. These studies will shed light on the molecular basis of neurodegenerative diseases, and provide guidelines for rational drug design and delivery in treating such disorders.
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