We demonstrated previously that forced expression of the neuronal phosphoprotein neuromodulin (also known as GAP-43, F1, B-50, and p57) in mouse anterior pituitary AtT-20 cells enhances depolarization-mediated secretion and alters cellular morphology. Here we analyze the role of calmodulin binding by neuromodulin in these responses. In cells expressing wild-type neuromodulin, a complex with calmodulin that is sensitive to intracellular calcium and phosphorylation is localized to the plasma membrane. Transfection of several mutant forms of neuromodulin shows that the effects of this protein on secretion are dependent on both calmodulin binding and association with the plasma membrane. In contrast, the morphological changes depend only on membrane association. Thus, the multitude of effects of neuromodulin noted in previous studies may result from divergent properties of this protein.The neuronal growth-associated protein neuromodulin (also designated GAP-43, B-50, and F1) is a membrane-bound phosphoprotein expressed at a high level during neuronal development and regeneration (reviewed in Refs. 1 and 2). Neuromodulin is a rapidly transported axonal protein (3-10) that is concentrated in the growth cone of elongating axons (8,(11)(12)(13)(14)(15)(16)(17). Additionally, overexpression of neuromodulin in the nervous system of transgenic mice causes spontaneous nerve sprouting at the neuromuscular junction and potentiates lesion-induced nerve sprouting and terminal arborization during re-innervation (18). Taken together, these results suggest that neuromodulin plays an important role in axon elongation. Further support for this notion has been derived from studies of several cell culture model systems (19 -28).However, a line of PC12 cells in which neuromodulin expression is nearly undetectable is nevertheless capable of robust neurite elongation in response to nerve growth factor (29). Additionally, cultured neurons derived from mice in which the neuromodulin gene has been disrupted by gene targeting extend axons to the same extent as cells that express this protein.Further analysis of the neuromodulin (Ϫ) embryos revealed that retinal axons appear incapable of crossing the midline decision point in the optic chiasm, implying that their growth cones fail to respond to environmental guidance cues (30).These results suggest that neuromodulin is perhaps not essential for axon elongation, but rather might function as a mediator of signal transduction pathways in the growth cone that serve to modulate the rate, extent, and trajectory of axonal growth.In the adult nervous system, neuromodulin expression persists in pre-synaptic terminals in those regions where the synaptic modifications associated with learning and memory are thought to occur (31-36). Additionally, the correlation of PKC 1 -mediated neuromodulin phosphorylation with long term potentiation of synaptic transmission in the hippocampus (37-40) and neurotransmitter release in vitro (41) suggest a role for neuromodulin in neuronal plasticity, possibly via modul...

The neuronal growth-associated protein (GAP)-43 (neuromodulin, B-50, F1), which is concentrated in the growth cones of elongating axons during neuronal development and in nerve terminals in restricted regions of the adult nervous system, has been implicated in the release of neurotransmitter. To study the role of GAP-43 in evoked secretion, we transfected mouse anterior pituitary AtT-20 cells with the rat GAP-43 cDNA and derived stably transfected cell lines. Depolarization-mediated beta-endorphin secretion was greatly enhanced in the GAP-43-expressing AtT-20 cells without a significant change in Ca2+ influx; in contrast, expression of GAP-43 did not alter corticotropin-releasing factor-evoked hormone secretion. The transfected cells also displayed a flattened morphology and extended processes when plated on laminin-coated substrates. These results suggest that AtT-20 cells are a useful model system for further investigations on the precise biological function(s) of GAP-43.