Signalling events regulating the retrograde axonal transport of I−βNerve growth factor in vivo

The molecular mechanisms regulating the retrograde axonal transport of nerve growth factor (NGF) are currently unknown. This study identifies some of the signalling events involved. The phosphoinositide 3-kinase (PI3-kinase) inhibitor wortmannin (1 nmol/eye) irreversibly inhibits the amount of ¹²⁵I−βNGF retrogradely transported in both sensory and sympathetic neurons. Another PI3-kinase inhibitor LY294002 (100 nmol/eye) also inhibited ¹²⁵I−βNGF retrograde transport in sensory neurons. The pp70^S6K inhibitor rapamycin (1 μmol/eye) had the same effect, inhibiting ¹²⁵I−βNGF transport only in sensory neurons. The cPLA₂ inhibitor AACOCF₃ (10 nmol/eye) had no effect on ¹²⁵I−βNGF transport in either sensory or sympathetic neurons. The TrkA receptor tyrosine kinase inhibitor AG-879 (10 nmol/eye) reduced ¹²⁵I−βNGF transport by approximately 50% in both sensory and sympathetic neurons. Cytochalasin D (2 nmol/eye), a disruptor of actin filaments and the dynein ATPase inhibitor erythro-9-[3-(2-hydroxynonyl)]adenine (EHNA) both inhibited ¹²⁵I−βNGF retrograde transport. These results demonstrate that in vivo TrkA tyrosine kinase activity, actin filaments and dynein are involved in the retrograde transport of NGF. In addition, different PI3-kinase isoforms may be recruited within different neuronal populations to regulate the retrograde transport of NGF. Potentially, these isoforms could activate alternative signalling pathways, such as pp70^S6K in sensory neurons.