Bezzerides et al. elucidated a signaling pathway whereby growth factors can elicit neurite outgrowth through an intriguing mechanism involving channel translocation. Calcium enters growth cones through an undefined population of channels to elicit calcium waves that regulate neurite outgrowth. The TRP (transient receptor potential) family of largely cation-nonselective channels includes likely candidates; however, the mechanisms that regulate the activity of these channels remain poorly understood (see Montell). Bezzerides et al. used evanescent field objective-based microscopy, surface biotinylation, and whole-cell patch analysis to show that epidermal growth factor stimulated translocation of TRPC5 from nearby vesicles into the plasma membranes of human embryonic kidney 293 (HEK293) cells expressing labeled TRPC5. The authors used a fluorescently labeled Akt domain as a biosensor for phosphatidylinositol-3,4,5-trisphosphate in combination with pharmacological analysis to implicate phosphoinositide 3-kinase (PI3K) in TRPC5 translocation. Dominant-negative Rac1 mutants blocked TRPC5 incorporation into the plasma membrane, whereas microinjection of constitutively active Rac1 stimulated TRPC5 insertion. Experiments involving expression of either the inhibitory domain of p21-activated kinase (PAK) or a phosphatidylinositol-4-phosphate 5-kinase (PIP5K) mutant indicated that PIP5K, but not PAK, acted downstream of Rac1 to mediate TRPC5 translocation. Various growth factors stimulated PI3K-dependent translocation in primary cultures of rat hippocampal neurons, which express endogenous TRPC5. Moreover, forced expression of PIP5K reduced neurite outgrowth, an effect that was overcome by a TRPC5 dominant-negative mutant. Thus, growth factors can promote calcium entry by stimulating TRPC5 translocation, a mechanism that the authors propose may be generally involved in regulating calcium influx and thereby calcium-dependent changes in cell morphology.
V. J. Bezzerides, I. S. Ramsey, S. Kotecha, A. Greka, D. E. Clapham, Rapid vesicular translocation and insertion of TRP channels. Nat. Cell Biol. 6, 709-720 (2004). [Online Journal]
C. Montell, Exciting trips for TRPs. Nat. Cell Biol. 6, 690-692 (2004). [Online Journal]