J. Neurochem. (2009) 112, 882–899.

Ectopic expression of the TrkA receptor tyrosine kinase in tumors of the nervous system can mediate nerve growth factor (NGF)‐dependent cell death by apoptosis and /or autophagy. Herein, we demonstrate that TrkA can also induce cell death in medulloblastoma Daoy cells by a caspase‐independent mechanism that involves the hyperstimulation of macropinocytosis. Specifically, NGF‐stimulates the uptake of AlexaFluor546‐dextran into lysosome‐associated membrane protein‐1 positive vacuoles which fuse with microtubule associated protein light chain 3 (LC3) positive autophagosomes, to form large intracellular vacuoles (> 1 μm), which then fuse with lysotracker positive lysosomes. While LC3 cleavage and the appearance of LC3 positive vacuoles suggest the induction of autophagy, siRNA reduced expression of four proteins essential to autophagy (beclin‐1, Atg5, LC3 and Atg9) neither blocks NGF‐induced vacuole formation nor cell death. TrkA activated cell death does not require p38, JNK or Erk1/2 kinases but does require activation of class III PI‐3 kinase and is blocked by the casein kinase 1 (CK1) inhibitor, D4476. This inhibitor does not interfere with TrkA activation but does block NGF‐dependent AlexaFluor546‐dextran uptake and CK1 dependent phosphorylation of β‐catenin. Collectively, these data demonstrate that TrkA stimulates cell death by a novel mechanism involving CK1‐dependent hyperstimulation of macropinocytosis.