Post by Deborah Joye
What's the science?
A stroke occurs when brain cells are not able to receive oxygen via blood flow due to a blockage or a burst blood vessel, and begin to die. In the first weeks to months after a stroke, many patients can exhibit functional improvements, though the chance of recovery depends on the size and location of the stroke event. Strokes can also damage cells that make up the corticospinal tract – long axons sent from the motor regions of the brain down to the spinal cord to innervate the muscles of the body. After stroke, the undamaged side of the spinal cord can sprout new axons, which can reinnervate the damaged region leading to some functional recovery. What is happening inside spinal cord cells that might trigger this rewiring after stroke? This week in The Journal of Neuroscience, Kaiser and colleagues use transcriptomic profiling to demonstrate that molecular changes in the stroke-affected spinal cord exhibit two distinct phases: an early inflammatory phase where microglia are activated in the damaged area, and a later growth-promoting phase involving sprouting of new axons and synapse formation.
How did they do it?
The authors induced photothrombotic stroke in mice by injecting them with a light-sensitive dye, then exposing part of the cortex to light, inducing stroke-like damage. Before and after the stroke, the authors tested mice to see how well they used their forelimb (i.e. front paw). To investigate how the undamaged side of the corticospinal tract might reinnervate the damaged spinal cord in space and time, the authors injected an anterograde tracer called Biotinylated dextran amines into the motor cortex in the brain. Injection of the tracer resulted in labelling of forelimb-related cells within the corticospinal tract and allowed for visualization and quantification of axonal sprouting. To identify the transcriptomic profile of these cells, the authors microdissected out the cells from the spinal cord at key time points (4, 7, 14, 28, and 42 days post-stroke) and put them through RNA-sequencing and analyses. The authors also assessed morphology and distribution of activated microglia — a sign of immune response in the brain. Finally, the authors assessed the role of potential growth-related genes by performing an in vitro neurite outgrowth assay where neurons are exposed to different signals and growth of new neurites is quantified.
What did they find?
Overall, the authors found that cells in the corticospinal tract undergo two different phases after stroke-induced cell death. The first phase occurs 4 to 7 days after the injury and is characterized by increases in inflammatory processes including activated microglia in the damaged region and phagocytic processes to clean up debris. The second phase occurs later, around one month post-injury, and is characterized by upregulation of growth-promoting factors, including neurite sprouting responses and synapse formation. Transcriptomic profiling revealed either upregulation or downregulation of 955 genes, with the most pronounced changes in gene expression seen at the 28-day timepoint. Of the upregulated genes associated with neurite growth, three of them were able to overcome growth inhibition signals typically present in the spinal cord and display increased growth in the neurite outgrowth assay.
What's the impact?
This is the first study to examine transcriptional changes in corticospinal neurons at multiple time points during recovery from stroke-related cell death. In addition to providing important insight into molecular changes that occur in spinal cord cells after stroke injury, the authors also reveal several factors that may serve as a basis for future neuroregenerative treatment options for stroke patients. Since there are currently few therapies for human patients with spinal cord injury, advances in this area have the potential to revolutionize clinical options for these patients.
Kaiser et al., The Spinal Transcriptome after Cortical Stroke: In Search of Molecular Factors Regulating Spontaneous Recovery in the Spinal Cord, Journal of Neuroscience (2019). Access the original scientific publication here.