Site Specific Response of Macrophages to Peripheral Nerve Injury

Overview

Macrophages infiltrate the dorsal root ganglia (DRG) and the sciatic nerve (SN) distal to the site of injury after peripheral nerve axotomy. Injury induced expression of the chemokine CCL2 in the DRG and SN attracts the macrophages to those sites. Macrophages accumulating in the ganglia and nerve have been shown to be beneficial for axon regeneration. However, the site-specific roles of macrophages have not been clearly determined, since most macrophage inhibition affects the cells at both sites. To inhibit macrophage accumulation at a specific site, the cre-lox system was used to knockout CCL2 in the DRG or SN by mating CCL2-floxed mice (CCL2fl/fl) with sensory neuron-expressing cre (Advillin-Cre) or Schwann cell-expressing cre (P0-Cre) mice, respectively. CCL2 mRNA expression was measured in the DRG and SN. Advillin-CreCCL2ﬂ/ﬂ mice showed significantly impaired CCL2 expression in both the DRG and SN, while P0-CreCCL2ﬂ/fl mice displayed impaired CCL2 expression in the SN only, compared to control mice. Macrophage accumulation was measured by staining for the marker CD68 in DRG and SN 7 days after SN transection injury. Interestingly, Advillin-CreCCL2ﬂ/ﬂ mice showed no change in macrophage accumulation in both the DRG and SN compared to controls, while P0-CreCCL2ﬂ/fl mice displayed impaired macrophage accumulation in the SN only. In an in vitro DRG culture, measuring axon regeneration, Advillin-CreCCL2ﬂ/ﬂ mice displayed significantly reduced neurite outgrowth compared to the other genotypes. Using this conditional knockout approach we will be able to delineate the contribution of macrophages in the SN and the DRG to the overall regenerative process.