Neuronal Growth Program

Neurons in cortex adjacent to the infarct can sprout new connections. This process is remarkable in cortex in the adult brain, as in most cases the connections of adult cortex are static. Sprouting neurons within the adult cortex activate neuronal growth-associated genes during phases of axonal sprouting after stroke.  However, the specific molecular growth program, or regeneration transcriptome, has until recently not been determined. This is in large part because the neurons that form a new connection after stroke are small in number and imbedded within cortical circuits in which most neurons do not engage in axonal sprouting. In order to identify the molecular growth program after stroke, the neurons that actually utilize this program must be isolated and studied. How does one do this? In other models of nerve regeneration, the regenerating neurons can be isolated with relative ease, such as in the dorsal root ganglion or the retina for the regenerating optic nerve. In cortex this has not possible without selectively labeling sprouting neurons after stroke. In past work, the Carmichael lab developed a way to do selectively label sprouting neurons after stroke with sequential injections of axonal tracers. Isolation and transcriptional profiling of neurons that form a new connection after stroke identified key intracellular molecules that produce epigenetic modifications in a growth program (ATRX), survival growth factors during recovery (IGF-1) and axonal sprouting growth factors (GDF10). Subsequent studies have identified GDF10 as a protein induced in rodent, primate and human stroke and that induces axonal sprouting in vitro and in vivo after stroke.

One ongoing project is defining other differentially regulated genes in neurons that develop a new connection after stroke, including apparently pro-growth extracellular matrix proteins and key transcription factors that might control axonal sprouting after stroke.
A second project is identifying the retrograde injury signal that is induced in neurons that projected to the stroke site and lost an axon or axon collateral. Our idea is that the retrograde injury signal will be a key event in this population of neurons that will influence their participation in other brain functional circuits, and may be a signal that transitions a neuron into a regenerative state. Our studies in this set of projects use mouse stroke models, scRNAseq, bioinformatics in this data analysis, viral gene expression and gain and loss of function studies and two-photon in vivo imaging.