Acute stroke produces a progressive loss of blood brain integrity, serum extravastation and inflammatory damage in the brain. Astrocytes react to this damage by producing molecules that lead to a scar and prevent stroke progression. These molecules are initially beneficial, as knocking out reactive astrocytosis leads to a larger area of damage and greater inflammation after acute brain or spinal cord injury. However, in the tissue repair and reorganization phase after stroke these glial scar-related molecules inhibit axonal sprouting. Most of the work on glial growth inhibitors has been done in spinal cord injury or in traumatic brain injury, such as in stab wounds. These studies have shown that chondroitin sulfate proteoglycans form a key component of the glial scar, and myelin-associated proteins provide a major growth blockade to axonal sprouting. This is analogous to a fig vine that was growing in a building near the Carmichael lab (the header picture above). One vine grows from the sunlight into the darkness, and then turns around to grow back to the light. One vine grows into the darkness and then its growing tip dies. The formation of new connections after CNS injury is inhibited by the environment of the injured brain. New connections are blocked from growing, and many collapse.W
We wanted to identify glial growth inhibitory molecules that are important in brain injury after stroke. We first screened the tissue that exhibits post-stroke axonal sprouting for candidate glial growth inhibitory proteins that were expressed at the right time to be in a position to block axonal sprouting. One gene that was induced during the first two weeks after the stroke, during the early phases of axonal sprouting, was EphrinA5. Importantly, EphrinA5 is further induced in the aged brain after stroke. EphrinA5 is an interesting molecule because it helps pattern the developing brain, and form the sensorimotor system. These results suggested that EphrinA5 might be re-activated after stroke in the sensorimotor system and limit axonal sprouting and recovery. Further, sprouting neurons in the aged animal activate a receptor for EphrinA5, EphA4, and a downstream signaling molecule for EphA4, chimaerin-1. In total these results position the EphrinA5 signaling system with both receptor and ligand specificity for axonal sprouting after stroke.
Ongoing studies in the lab have used laser capture microdissection of reactive astrocytes to determine if these are the cells that express the elevated EphrinA5 levels after stroke, and have used gain and loss of function experiments within EphrinA5 signaling to determine axonal sprouting and motor recovery after stroke in mice. These studies have been led by Dr. Justin Overman in the lab and are submitted.
Future work will determine the interactions of EphrinA5 signaling with behavioral activity after stroke, and the role of other candidate glial growth inhibitors in post-stroke axonal sprouting and recovery.