Precursors of the olfactory interneurons migrate from the subventricular zone via the rostral migratory stream (RMS). To investigate the molecular mechanisms by which RMS cells migrate, we used a slice preparation, which allows the migrating cells to be imaged at very high temporal and spatial resolution in the presence of added inhibitors. Using immunohistochemistry, we first determined that the ␣1-, ␤8-, and ␤1-integrin subunits and the ␣5-and ␥1-laminin subunits are expressed during embryonic day 16 to the early postnatal stage. During early postnatal days, ␣v-and ␤6-integrins appeared, and their expression persisted throughout adulthood. The migrating cells also expressed the netrin receptors neogenin and Deleted in Colorectal Carcinoma (DCC). Netrin-1 is expressed in olfactory mitral cells. Anti-integrin antibodies inhibited the production of protrusions as well as cellular translocation. In contrast, anti-DCC antibodies primarily altered the direction of the protrusions; consequently, the migration was no longer unidirectional, and the speed was reduced. Thus, the interaction of DCC, possibly through an interaction with netrin-1, contributes to the direction of migration by regulating the formation of directed protrusions. In contrast, the integrins function in production of protrusions and cellular translocation, with different integrins participating at different developmental stages.

Microglia rapidly become reactive in response to diverse stimuli and are thought to be prominent participants in the pathophysiology of both acute injury and chronic neurological diseases. However, mature microglial reactions to a focal lesion have not been characterized dynamically in adult vertebrate tissue. Here, we present a detailed analysis of long-distance perilesional microglial migration using time-lapse confocal microscopy in acutely isolated living slices from adult brain-injured mice. Extensive migration of perilesional microglia was apparent by 24 h after injury and peaked at 3 d. Average instantaneous migration speeds of ϳ5 m/min and peak speeds Ͼ10 m/min were observed. Collective, directed migration toward the lesion edge was not observed as might be expected in the presence of chemoattractive gradients. Rather, migration was autonomous and could be modeled as a random walk. Pharmacological blockade of the cysteine-cysteine chemokine receptor 5 reduced migration velocity and the number of perilesional migratory microglia without affecting directional persistence, suggesting a novel role for chemokines in modulation of discrete migratory parameters. Finally, activated microglia in the denervated hippocampal stratum oriens did not migrate extensively, whereas human immunodeficiency virus-1 tat-activated microglia migrated nearly twice as fast as those at the stab lesion, indicating a nonuniform microglial response to different stimuli. Understanding the characteristics and specific molecular mechanisms underlying microglial migration after neural injury could reveal novel targets for therapeutic strategies for modulating neuroinflammation in human diseases.

To investigate whether the c-fms proto-oncogene plays a role in the CNS, we examined its expression in mouse brain. We found that c-fms-positive Purkinje cells first appeared in caudal cerebellum at postnatal day 0 (P0) arranged in a parasagittal manner, and most Purkinje cells gradually became positive by P6. This differential expression was not seen from P7 to adulthood, and the parasagittal pattern until P5 was different from those of L7, zebrins, and the integrin beta1 subunit. No neuronal expression of c-fms was found in the other brain regions examined. In both reeler and weaver mutant mice in the adult stage, all Purkinje cells were positive for c-fms as in the wild-type controls; however, the parasagittal bands of c-fms-positive Purkinje cells were observed even in the adult staggerer mutant. To check the neurotrophic effect of macrophage colony-stimulating factor (M-CSF), we immunostained cerebella derived from osteopetrotic mutant mice, that is, those devoid of active M-CSF. We found that the number of calbindin-positive Purkinje cells in a given cerebellum began to decrease substantially during the initial 4-5 weeks of the postnatal period. In addition, cultured Purkinje cells were dependent on M-CSF for their survival. These data suggest that expression of the c-fms gene is intrinsically programmed in the Purkinje cells and never affected by the afferent synaptic input and that neuronal survival of Purkinje cells is dependent on M-CSF after weaning. Therefore, c-fms is considered to be a new developmental marker for Purkinje cells.

At the premyelinating stage, the Schwann cells of peripheral nerves are able to recognize the axon, to arrange themselves along it in a nonoverlapping manner, and finally to establish a one-to-one cell-axon relationship. The mechanism that regulates these processes is not known in detail. We found the existence of a significant Schwann cell apoptosis in vivo of rat postnatal sciatic nerve, peaking around postnatal day 3. More than 50% of the neonatal Schwann cells cultured in axon-free medium undergo a rapid apoptosis. The apoptosis can be suppressed by addition of survival factors such as Neu differentiation factors or by increasing the adhesion of Schwann cells to substratum. We suggest that in neonatal nerves in vivo, Schwann cells are highly susceptible to apoptosis, but they are saved from death by contact with axons. The dramatic increase in number of Schwann cells between postnatal day 0 and 3 overcomes the number of axons available for them. Consequently the Schwann cells that fail to contact an axon undergo apoptosis. In conclusion, the number of Schwann cells in the developing nerves is regulated by the apoptosis and clearly depends on the survival signals from axons.

Neuroblasts migrate from the subventricular zone along the rostral migratory stream (RMS) to the olfactory bulb (OB). While the migration occurs by movement over other cells, the molecular mechanisms are poorly understood. We have found that ADAM2 (a disintegrin and metalloprotease 2) is expressed in migrating RMS neuroblasts and functions in their migration. The brains from ADAM2 knockout (KO) mice showed a smaller OB than that seen in wild-type (WT) mice at postnatal day 0. In addition, the RMS in ADAM2 KO mice appeared thinner and less voluminous in its rostral part and thicker in its caudal part. Estimates of migration in vivo using bromodeoxyuridine labeling revealed that neuroblasts from KO mice show a decreased migration rate compared with those from WT mice. Direct assays of migration by imaging living slices also showed a decreased migration speed and loss of directionality in the KO mice. This phenotype was similar to that seen in RMS containing slices from WT mice exposed to a peptide that mimicked the disintegrin loop of ADAM2. Finally, RMS explants from KO or WT mice that were cultured in Matrigel also revealed striking differences. The cells migrating out of explants from WT mice showed robust cell-cell interactions. In contrast, fewer cells migrated out of explants from ADAM2 KO mice, and those that did were largely dispersed and their migration inhibited. These experiments suggest that ADAM2 contributes to RMS migration, possibly through cell-cell interactions that mediate the rapid migration of the neuroblasts to their endpoint.