The cells arising in the anterior part of the subventricular zone (SVZa) migrate along a well-demarcated pathway which lacks radial glial fibers to the olfactory bulb where they differentiate into interneurons of the granule cell layer or glomerular layer (Luskin, 1993, Neuron 11, 173). To analyze the mechanisms underlying this highly directed migration, we have compared the migratory behavior of unmanipulated SVZa-derived cells to that of homotopically transplanted SVZa cells and of heterotopically transplanted telencephalic ventricular zone (VZ) cells that ordinarily migrate in association with radial glial fibers. To identify the phenotype of the SVZa progenitor cells prior to their transplantation, we characterized them in vitro using cell type-specific markers. After 1 day in culture nearly all the SVZa cells were stained with TuJ1, a neuron-specific marker; only an occasional cell exhibited a glial phenotype as judged by the presence of GFAP-immunoreactivity. This indicates that SVZa cells express a neuronal phenotype. To reveal the spatiotemporal distribution of homotopically transplanted neonatal SVZa cells in a host brain, dissociated SVZa cells from Postnatal Day 0 (P0)-P2 animals were labeled with the lipophilic dye PKH26 or the cell proliferation marker BrdU and implanted into the SVZa of host animals of the same age. Within the first week after transplantation there were vast numbers of labeled cells throughout the pathway. Over the next 2 weeks the labeled cells migrated into the overlying cellular layer of the olfactory bulb and began to differentiate, and within 4 weeks the transplanted cells had reached their final positions in the granule cell and glomerular layers of the olfactory bulb in the same proportions as for unmanipulated SVZa-derived cells. While en route to the olfactory bulb the homotopically transplanted cells never strayed from the migratory pathway. In contrast, heterotopically transplanted VZ cells from the embryonic telencephalon did not undergo migration although they did differentiate. These results demonstrate that the homotopically transplanted SVZa-derived cells adopt a mode of migration indistinguishable from that ordinarily utilized by SVZa-derived neurons and that the VZ cells are unable to decipher the same set of guidance cues.

Differential targeting of neuronal proteins to axons and dendrites is essential for directional information flow within the brain, however, little is known about this protein-sorting process. Here, we investigate polarized targeting of lipid-anchored peripheral membrane proteins, postsynaptic density-95 (PSD-95) and growthassociated protein-43 (GAP-43). Whereas the N-terminal palmitoylated motif of PSD-95 is necessary but not sufficient for sorting to dendrites, the palmitoylation motif of GAP-43 is sufficient for axonal targeting and can redirect a PSD-95 chimera to axons. Systematic mutagenesis of the GAP-43 and PSD-95 palmitoylation motifs indicates that the spacing of the palmitoylated cysteines and the presence of nearby basic amino acids determine polarized targeting by these two motifs. Similarly, the axonal protein paralemmin contains a C-terminal palmitoylated domain, which resembles that of GAP-43 and also mediates axonal targeting. These axonally targeted palmitoylation motifs also mediate targeting to detergent-insoluble glycolipid-enriched complexes in heterologous cells, suggesting a possible role for specialized lipid domains in axonal sorting of peripheral membrane proteins.Proper neuronal function requires selective protein targeting to specialized cellular and plasma membrane domains including the nerve terminal, node of Ranvier, axon hillock, and postsynaptic density. An early step in this targeting decision tree involves a polarized sorting of proteins to either dendritic (postsynaptic) or axonal (presynaptic) domains. However, the mechanisms by which neurons target specific proteins to dendrites versus axons are poorly understood. Better characterized is protein sorting to apical versus basolateral plasma membranes in polarized epithelial cells, which share certain features with axonal versus dendritic targeting in neurons (1, 2). That is, short cytosolic C-terminal protein-sorting motifs are one route for both dendritic and basolateral targeting (2), whereas specialized lipid rafts can mediate both axonal and apical sorting of certain transmembrane and glycosylphosphatidylinositol-anchored membrane proteins (3).The concept of specialized lipid rafts mediating polarized protein targeting emerged from observations that apical and basolateral cell membranes have different lipid compositions. Apical membranes are enriched in sphingolipids that aggregate with cholesterol to form packed raft-like domains within the fluid membrane bilayer. These rafts are insoluble in nonionic detergents and, hence, are termed detergent-insoluble glycolipid-enriched complexes (DIGs).1 These complexes form in the trans-Golgi network and incorporate certain transmembrane, GPI-anchored, and dually acylated proteins, which are then targeted to the apical plasma membrane (4, 5). The inhibition of DIG formation by sphingolipid or cholesterol depletion disrupts this apical/axonal sorting pathway (3, 6, 7). However, the polarized targeting of cytosolic proteins via DIGs has not been explored.Postsynaptic density-95 (PSD-9...

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