Researchers here report on a mechanism that increase the regenerative capacity of brain cells following the damage of a stroke, at least in mice. There are now a few similar approaches demonstrated in the laboratory, but it remains to be seen whether any of them will lead to therapies in the near future. It is certainly the case that mammalian cells do not respond to structural damage and loss of blood supply in the most optimal way; many of their reactions make matters worse, not better. Perhaps that can be adjusted safely and soon, though it would be far preferable to focus on potential ways to prevent that sort of event from occurring at all, such as better maintenance of blood vessels, control of atherosclerosis, and the like.
Stroke is a leading cause of death and chronic disability in adults, causing a heavy social and economic burden worldwide. However, no treatments exist to restore the neuronal circuitry after a stroke. The mammalian brain has only a limited ability to regenerate neuronal circuits for functional recovery. While most neurons are generated during embryonic brain development, new neurons continue to be produced in the ventricular-subventricular zone (V-SVZ) of the adult brain.
In a rodent ischemic stroke model induced by transiently blocking the middle cerebral artery, the most commonly affected vessel in human patients, some V-SVZ-derived neuroblasts migrate toward the lesion, where they mature and become integrated into the neuronal circuitry. However, the number of these new neurons is insufficient to restore neuronal function. Within a few days after stroke, astrocytes, a major population of macroglia, in and around the injured area become activated, exhibiting larger cell bodies, thicker processes, and proliferative behavior. The migrating neuroblasts must navigate through this astrocyte meshwork to reach the lesion.
The research team demonstrated that neuroblast migration is restricted by the activated astrocytes in and around the lesion. In normal, olfaction-related migration, neuroblasts secrete a protein called Slit, which binds to a receptor called Robo expressed on astrocytes. Slit alters the morphology of activated astrocytes at the site of neuroblast contact, to move the astrocyte surface away and clear the neuroblast's migratory path. However, in the case of brain injury, the migrating neuroblasts actually down-regulated their Slit production, crippling their ability to reach the lesion for functional regeneration. Notably, overproducing Slit in the neuroblasts enabled them to migrate closer to the lesion, where they matured and regenerated neuronal circuits, leading to functional recovery in the post-stroke mice.