Growth cone advance mediated by fibronectin-associated filopodia is inhibited by a phorbol ester tumor promoter

In serum-supplemented medium, exposure to the tumor promoter 4β-phorbol 12β-myristate 13α-acetate (PMA) increases the proportion of SH-SY5Y neuroblastoma cells with neurites and increases the average neurite length. In the present study, under serum-free conditions, PMA treatment had the opposite effects, i.e., retarded neurite sprouting and partially inhibited neurite elongation. This inhibition in neurite outgrowth was partially antagonized by the addition of serum fibronectin (FN) to the medium or substratum. In the absence of PMA, SH-SY5Y cells grown under serum-free conditions showed extensive neurite outgrowth as well as the capacity to secrete FN into their microenvironment and form FN-containing substratum-attachment sites. Immunogold labeling and whole mount transmission electron microscopy (WMTEM) demonstrated FN-containing contact pads at sites where filopodia attached to the substratum and focal plaques on the underside of growth cone margins. The appearance and abundance of FN-containing contact pads and focal plaques were increased by the addition of exogenous FN to defined medium. Focal plaques appeared in close association with microfilament bundles, and nearly always with bundles that projected into filopodia attached to the substratum by contact pads. A method for immunolabeling FN in the filopodial contact pads of living cultures provided more direct evidence that filopodia and contact pads have a major role in FN-mediated attachment and are central in determining growth cone shape and the rate and direction of advance. In support of this view, we show that PMA treatment retards neurite sprouting, alters growth cone morphology and motility, and eliminates the appearance of microfilament bundles, filopodia, and FN-containing substratum-attachment plaques.