Gene expression changes of interconnected spared cortical neurons 7 days after ischemic infarct of the primary motor cortex in the rat.

Urban ET 3rd, Bury SD, Barbay HS, Guggenmos DJ, Dong Y, Nudo RJ.

Abstract

After cortical injury resulting from stroke, some recovery can occur and may involve spared areas of the cerebral cortex reorganizing to assume functions previously controlled by the damaged cortical areas. No studies have specifically assessed gene expression changes in remote neurons with axonal processes that terminate in the infarcted tissue, i.e., the subset of neurons most likely to be involved in regenerative processes. By physiologically identifying the primary motor area controlling forelimb function in adult rats (caudal forelimb area = CFA), and injecting a retrograde tract-tracer, we labeled neurons within the non-primary motor cortex (rostral forelimb area = RFA) that project to CFA.

Then, 7 days after a CFA infarct (n = 6), we used laser capture microdissection techniques to harvest labeled neurons in RFA. Healthy, uninjured rats served as controls (n = 6). Biological interactions and functions of gene profiling were investigated by Affymetrix Microarray, and Ingenuity Pathway Analysis. A total of 143 up- and 128 down-regulated genes showed significant changes (fold change ≥1.3 and p < 0.05).

The canonical pathway, “Axonal Guidance Signaling,” was overrepresented (p value = 0.002). Significantly overrepresented functions included: branching of neurites, organization of cytoskeleton, dendritic growth and branching, organization of cytoplasm, guidance of neurites, development of cellular protrusions, density of dendritic spines, and shape change (p = 0.000151-0.0487). As previous studies have shown that spared motor areas are important in recovery following injury to the primary motor area, the results suggest that these gene expression changes in remote, interconnected neurons may underlie reorganization and recovery mechanisms.

Additional description:

After clinical stroke, it is now widely held that brain regions spared by the injury reorganize both structurally and functionally, presumably supporting recovery of motor behavior. The spared brain areas that are most likely to reorganize are those that are anatomically connected with the ischemic core. For example, after a middle cerebral artery infarct, the primary motor cortex is often injured. Premotor brain areas have extensive reciprocal connections with the primary motor cortex, and have been implicated in recovery.

Prior studies by neuroscientists at the University of Kansas Medical Center have shown that premotor areas reorganize functionally, as demonstrated by expansion of motor maps. The axons of these premotor area neurons sprout to form corticocortical connections with a new set of distant brain regions. In this new study from the University of Kansas Medical Center, Urban et al., used a unique approach to pre-label neurons that project to the primary motor cortex in rats.

Then, after an ischemic infarct in the primary motor cortex and sufficient survival time, they harvested the labeled neurons using laser-capture microdissection. Gene expression profiles and canonical pathway analysis demonstrated significantly upregulated functions related to axonal growth and guidance. The identification of these genes and their associated pathways may lead to a new set of targets for development of a new class of recovery drugs for future brain repairstrategies

Figure caption. Schematic drawing of the events that take place after an ischemic stroke in an experimental rat model. Occlusion of blood vessels supplying the motor cortex results in a necrotic core (grey area). Neurons in the infarct core normally send axons to the spinal cord to innervate motor neurons, that, in turn, project to muscle fibers (forearm neurons and muscles shown).

Weakness and reduction in skilled movements results. Neurons in spared premotor regions that normally send corticocortical fibers to the ischemic core (black triangle to the right of the grey area) survive the infarct. But their fibers that terminate in the ischemic core prune back and then sprout to connect to new areas. Genes that are involved in the axonal growth and guidance processes can be identified using involving a combination of neuronal tract-tracking, laser-capture microdissection and gene profiling techniques.

Gene expression changes of interconnected spared cortical neurons 7 days after ischemic infarct of the primary motor cortex in the rat.