Expression of a neuromodulin-beta-galactosidase fusion protein in primary cultured neurons and its accumulation in growth cones.

Cultured embryonic neurons share a number of characteristic morphological and physiological properties with their counterparts in vivo. For example, differentiating hippocampal neurons in culture develop two distinct classes of processes that serve as dendrites and axons. It has also been shown that the microtubule organization and composition in axons differs from those in dendrites, which may contribute to differential transport of macromolecules into axons or dendrites. We have expressed a neuromodulin-beta-galactosidase fusion gene in cultured mesencephalic neurons in order to study the transport of the neurospecific protein neuromodulin into neurite growth cones. When beta-galactosidase alone was expressed in neurons, it was found in the cell bodies with diffuse neurite staining. In marked contrast, the neuromodulin-beta-galactosidase fusion protein was rapidly transported into neurites and was concentrated in the growth cones. This system may provide a useful model for studying the structural domain(s) of neuromodulin that are required for transport and accumulation of neuromodulin in the growth cones of neurons.