The journey undertaken by newly generated neurons in the adult brain is like the cellular equivalent of the arduous upstream migration of salmon returning to the rivers in which they were hatched. Soon after they are born in the subventricular zone near the back of the brain, these cells migrate to the front-most tip of of the olfactory bulb. This is the furthest point from their birth place, and they traverse two-thirds of the length of the brain to get there.
The first leg of this epic journey – the departure of the newborn cells from the subventricular zone – involves some of the signalling cues that guide cell migrations during development of the brain. However, these signals alone are known to be insufficient, and until now the precise mechanisms governing this migration were unclear. But a new study by Canadian researchers shows that the cells travel such long distances by crawling along the capillaries in the olfactory bulb.
Cell migration is a key feature of nervous system development, and the mechanisms underlying these movements are well characterised. As a general rule, migrating cells rely on the combined activity of chemical signals to find their way. Thus, they set off on their migration because of a signal which repels them from their birth place and stay on track because their migratory pathway is flanked by a non-permissive signal which prevents them from deviating from the correct route. Finally, as they approach the end of their journey, attractive cues pull them in the right direction. Upon arriving at their destination, the cells begin to extend axons and dendrites, whose paths are guided by the same kinds of cues.
In the adult brain, cells born in the subventricular zone enter the olfactory bulb, where they are ensheathed by astrocytic cells before migrating in chains along it through the rostral migratory stream. As they reach the tip of the bulb at the front of the brain, they turn and exit the migration path one by one, then settle in and begin to differentiate into mature interneurons. The tissues on either side of the rostral migratory to secrete Slit, a repulsive axon guidance molecule which prevents migrating cells from veering out of their path, and the olfactory bulb itself secretes attractive cues which keep them on track. But the cell migrations still occur when the bulb itself is surgically removed from the brains of mice, so the chemical signals within it are clearly not essential.
Marina Snapyan of the Centre de Recherche Université Laval Robert-Giffardand in Québec and her colleagues therefore explored the possibility that the neuroblast migration is guided by some physical substrate which delineates the rostral migratory stream. First, they spliced the gene encoding green fluorescent protein (GFP) into a retovirus, and then pumped the virus into the subventricular zone or rostral migratory stream, so that it would be taken up by the migrating cells. They then labelled the blood vessels in the brains of these animals by injecting a fluorescent dye into the tail.
This showed that the blood vessels in the olfactory bulb were arranged in parallel to the rostral migratory stream, and that almost all the migrating neurons were aligned along the vessels. Furthermore, the vessels at the tip of the bulb were found to be arranged perpendicular to the migration stream. The researchers then prepared slices of forebrain tissue from the animals and examined the cell migration in real time using time-lapse video imaging.
They found that the neuroblasts do indeed migrate along the capillaries running through the olfactory bulb. The vast majority of cells remained closely attached to the vessels for the duration of their migration, and never strayed more than three thousandts of a millimeter away from them. In the few cases when the cells moved further away from the vessel, their leading processes remained attached to it. And once the cells exited the migratory stream at the tip of the bulb, they quickly latched onto the perpendicularly oriented vessels to migrate across the bulb’s radius.
Previous work has shown that the endothelial cells lining blood vessel walls synthesize a small protein called brain-derived neurotrophic factor (BDNF), and that injection of this protein into the ventricles of the brain leads to the appearance of new neurons in the striatum, a structure which migrating neuroblasts pass en route to the tip of the olfactory bulb. Snapyan and her colleagues therefore investigated whether BDNF mediates the imteraction between the neuroblasts and blood vessels. Using antibody staining, they anlaysed the distribution of BDNF and its receptors.
This revealed that BDNF is expressed exclusively in the endothelial cells of the blood vessels aligned with the rostral migraptory stream, and that migrating cells express one of its receptors, and the astrocytes which ensheath them during their migration express another. Furthermore, when the protein was injected into the striatum, numerous GFP-labelled neuroblasts were found there three days later – misexpression of BDNF had misrouted the cells from their normal migratory pathway. On the other hand, antibodies which neutralize the activity of the proteins, into the olfactory bulb led to a marked reduction in the number of neuroblasts migratory stream. Antibodies which neutralized p75NTR or TrkB had the same effect.
This is compelling evidence that newborn neurons generated in the subventricular zone of the adult brain migrate the long distance to the tip of the olfactory bulb by crawling along capillaries, and that this migration is mediated by BDNF and its receptors. The researchers confirmed their findings in a pièce de résistance: they engineered an artifical three-dimensional capillary network using endothelial cells and fibroblasts (the cells which are the main constituent of connective tissue), placed it in a culture dish, and added GFP-labelled neuroblasts isolated from their mice. Time-lapse imaging showed that the neuroblasts attached themselves to the artificial capillaries and migrated along them, and that migration was prevented by blocking the interaction of BDNF with its receptors, using either anitbodies or RNA interference.
Snapyan, M., et al (2009). Vasculature Guides Migrating Neuronal Precursors in the Adult Mammalian Forebrain via Brain-Derived Neurotrophic Factor Signaling. J. Neurosci. 29: 4172-4188. DOI: 10.1523/JNEUROSCI.4956-08.2009.