THE DUAL-SPECIFICITY CLK KINASE INDUCES NEURONAL DIFFERENTIATION OF PC12 CELLS

CLK is a dual-specificity protein kinase capable of phosphorylating se rine, threonine, and tyrosine residues. We have investigated the actio n of CLK by establishing stable PC12 cell lines capable of inducibly e xpressing CLK. Expression of CLK in stably transfected PC12 cells mimi cked a number of nerve growth factor (NGF)-dependent events, including the morphological differentiation of these cells and the elaboration of neurites. Moreover, CLK expression enhanced the rate of NGF-mediate d neurite outgrowth of these cells, indicating that CLK expression and NGF treatment activate similar signal transduction pathways. CLK expr ession, unlike NGF, was not able to promote PC12 cell survival in seru m-free media, demonstrating that CLK only partially recapitulated the actions of NGF on these cells and that the biochemical pathways necess ary for morphological differentiation can be stimulated without also s timulating those necessary for survival. Induction of CLK expression a lso resulted in the selective activation of protein kinases that are c omponents of growth factor-stimulated signal transduction cascades, in cluding ERK1, ERK2, pp90(RSK), and S6PKII. Induction of CLK expression , however, did not stimulate pp70(S6K) or Fos kinase, two NGF-sensitiv e protein kinases. These data indicate that CLK action mediates the mo rphological differentiation of these cells through its capacity to ind ependently stimulate signal transduction pathways normally employed by NGF.