First-author Florence Bareyre
A tragic truth is that the mammalian central nervous system (CNS) has very limited ability to regenerate following injury. There is intense interest in trying to understand these limits, so that they can be circumvented, thereby providing hope to victims of spinal cord and brain injuries. Many studies with these aims make use of a particular set of axons in the spinal cord called the corticospinal tract (CST), which carries information from the cerebral cortex to neurons in the spinal cord that then cause muscles to move. The CST has a clear-cut function and is often injured in humans. In experimental animals, however, it has only been possible to label the CST by injecting tracer molecules into the brain and then following their passage into the spinal cord. This technique has been useful but clearly needs improvement: (1) It requires surgery, rendering experimentation cumbersome. (2) It is difficult to perform on small animals. (3) It is incomplete, often failing to reveal the so-called minor CST components that are spared by injury and contribute to recovery. (4) It is variable from animal to animal, so large numbers of animals need to be studied to obtain reliable results. (5) It is incompatible with dynamic imaging in live preparations.
To circumvent these limitations, we have developed a new method to selectively and completely label the CST in mice. We used mice in which a yellow fluorescent protein (YFP) accumulates in CST axons in the spinal cord, rendering them visible under appropriate illumination. The method uses mice we previously developed, called “STOP-YFP,” in which YFP is expressed only in cells that are treated with an enzyme called “cre.” Cre is delivered by mating the STOP-YFP mice to another transgenic line in which cre is selectively expressed in the cortex. We then study the offspring that have both transgenes; in them, YFP is expressed in neurons of the cortex. As the CST is the only direct projection from cortex to spinal cord, this leads to specific, complete labeling of this tract in double transgenic mice, which we call CST-YFP.
Using CST-YFP mice, we found that (1) CST axons extend through the spinal cord earlier in development than previously reported; (2) the CST includes minor subpopulations of fibers that are often spared when the main population is cut; (3) this distinct, little-studied subpopulation of <10% of all CST axons accounts for a vast majority (~90%) of the direct CST input to motor neurons in normal animals; and (4) axons of this minor subpopulation form new synapses on motor neurons following injury to the major population. Together, these results provide new data on the structure and development of an important model system for analysis of spinal cord pathology; document a compensatory reorganization that may contribute to functional recovery following injury; and demonstrate the general utility of CST-YFP mice for spinal cord injury research.