Abstract 58: Neuronal Activation of Ras-Related C3 Botulinum Toxin Substrate 1 (Rac1) Improves Post-Stroke Recovery and Axonal Plasticity in Mice

Abstract

Axonal plasticity is critical for functional recovery after stroke. Our previous study suggested that Rac1, a small GTPase, contributes to post-stroke axonal regeneration. However, the specific cellular and molecular mechanisms mediating its effects remain unknown. The two major factors controlling axonal plasticity in the injured brain are the neuronal intrinsic property for outgrowth and astrocytic inhibition of axonal growth. We investigated the selective roles of neuronal and astrocytic Rac1 in post-stroke recovery and axonal regeneration. Stroke was induced by middle cerebral artery occlusion for 60 minutes in 7 to 8 week old male mice. Brain injection of lentivirus encoding Rac1 with neuronal (NSE) or astrocytic (GFAP) promotors was performed 7 days after stroke in wild type mice to overexpress neuronal or astrocytic Rac1. Tamoxifen administration was started 7 days after stroke in Thy1-cre/Rac1 floxed mice to induce genetic deletion of neuronal Rac1. Delayed overexpression of neuronal Rac1 promoted sensorimotor recovery assessed using a single pellet reaching test (% success rate) from 21 to 28 days after stroke (Day 28: GFP control 51.25±4.41 vs. NSE-Rac1 66.67±2.36, n=8-9, p<0.05). Memory and cognitive function was improved after stroke as assessed by the novel object recognition test (Day 28: GFP control 1.43±0.34 vs. NSE-Rac1 2.25±0.23, n=8, p<0.05). Consistently, delayed neuronal Rac1 deletion worsened recovery assessed by pellet reaching test (WT 60±4.08 vs. KO 42.5±2.5, n=4, p<0.05), an adhesive removal test (seconds) (WT 22.67±3.71 vs. KO 70.92±15.56, n=4, p<0.05), and novel object recognition test (WT 1.59±0.17 vs. KO 1.08±0.09, n=4, p<0.05) 28 days after stroke. Interestingly, astrocytic Rac1 overexpression did not affect functional recovery (not shown). Finally, overexpression of neuronal Rac1 increased axonal density (neurofilament-L staining) in the peri-infarct zone 28 days after stroke (GFP control 1.00±0.09 vs. NSE-Rac1 2.86±0.41, n=4, p<0.05), with no effect on brain cavity size. In conclusion, neuronal Rac1 signaling enhances axonal plasticity and improves post-stroke functional recovery. Neuronal specific targeting of Rac1 may have therapeutic potential for reducing long-term disability after stroke.