Neuronal precursor cells persist in the adult vertebrate forebrain and thus new neurons are continuously added to restricted regions, such as the olfactory bulb and hippocampus in mammals. Consequently, neuronal migration is not restricted to the embryonic milieu but also exists in an adult brain environment. Because the latter is thought to be largely a nonpermissive territory for cell movement - with the obvious exception of cancer cells -migration is restricted to very specific permissive pathways within the brain.
Perhaps the best-known example of adult neuro-genesis and neuronal migration is among the first discovered, that of the adult songbird forebrain (Goldman and Nottebohm, 1983). Songbirds display widespread neurogenesis and migration during adulthood, most remarkably in an area of the telencephalon involved in song learning, the higher vocal center (HVC). Studies carried out by Alvarez-Buylla and colleagues (Alvarez-Buylla and Nottebohm, 1988) showed that neurons originating in the SVZ migrate to the cortex when new neurons are added to the songbird hippocampus and HVC, in a process involving the guidance of radial fibers. Moreover, both diffusible and substrate-bound molecules control this migration through a set of hormonally regulated short-distance cell-cell interactions.
In contrast to the relatively widespread neurogen-esis found in songbirds, the adult mammalian forebrain utilizes progenitors to generate new neurons destined for very few regions. Specifically, the SVZ of the lateral ventricle and the dentate gyrus subgranular zone (SGZ) of the hippocampus are the regions where adult neurogenesis has been demonstrated (reviewed in Gage, 2000; Alvarez-Buylla and Lim, 2004). The adult SVZ produces new GABAergic interneurons for the olfactory bulb, whereas the SGZ gives rise to granule cells of the hippocampus. Despite the hostile territory that the adult brain represents for migration, new neurons in the hippocampus have a relatively easy path to their final destination, because they are very close to their final location. A different case is the migration of olfactory interneurons, which need to navigate through an extremely long distance from the SVZ to the olfactory bulb.
In contrast to the findings regarding neurogenesis and neuronal migration in songbirds, radial glia cells do not guide the postnatal migration of newly born cells. Instead, olfactory interneurons migrate using a cellular process called chain migration, which involves homotypic interactions between the migrating cells and tubular structures formed by specialized astrocytes (Lois etal., 1996). This migration occurs through a highly restricted route termed the rostral migratory stream (RMS). Like other cell populations in the embryonic brain, tangentially migrating olfactory interneuron precursors change the direction of movement on arriving in the olfactory bulb, migrating radially into specific layers.
Defining the diffusible or membrane-bound factors that guide the tangential migration of new interneurons from the adult SVZ to the olfactory bulb is a very active field in developmental neuro-biology. A polysialated glycoprotein neuronal cell adhesion molecule (PSA-N-CAM) is highly expressed on the surface of olfactory migrating neurons and it has been shown that deletion of the gene for N-CAM or enzymatic removal of PSA results in deficits in the migration of olfactory interneurons and a reduction in the size of the olfactory bulb (Cremer et al., 1994; Ono et al., 1994). Evidence suggests that PSA and/or N-CAM may not be essential for chain formation but, without them, there are several alterations in the nature of the chains that may inhibit the migration of neuronal precursors (Hu et al., 1996). Several additional adhesion molecules have been identified in the migratory route of olfactory interneuron precursors. For example, Tenascin-C, a ligand for avp3 and avp6 integrins, is strongly expressed in the astrocytes that form the tubes through which olfactory precursors migrate in the RMS (Jankovski and Sotelo, 1996), and av-, |33-, and p6-integrin subunits are also present in the postnatal RMS. Nevertheless, the lack of abnormalities in the olfactory bulb of mice with individual mutations for some of these molecules prevents a more definitive evaluation of the function of these proteins in vivo.
The molecular mechanisms guiding the highly directed migration of olfactory interneurons in the RMS are still unclear, although both attractive and repulsive guidance cues have been proposed to mediate this process. Among the repellents, Slit proteins have been shown to repel SVZ-derived cells in vitro (Hu, 1999; Wu et al., 1999). In addition, evidence demonstrates that the activation of the receptor tyr-osine kinase ErbB4 is essential for regulating the organization of neural chains in the RMS and therefore their migration (Anton et al., 2004). It seems evident that other molecules are likely to be involved in this process - future experiments will determine their molecular nature.
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