Background: Neuroimmune modulation following traumatic stress is accompanied by cortical upregulation of c-Src expression, but the mechanistic details of the potential regulatory link between c-Src expression and immunosuppression have not been established.
Methods: We used a combination of techniques to measure temporal changes in: (i) the parallel expression of c-Src and microRNA222; (ii) levels of PAK1 (p21-activated kinase 1); and (iii) the association between PAK1 and interleukin 1β signaling, both in cortex of rats following traumatic stress and in primary cortical neurons. Techniques included real-time PCR, immunoprecipitation, western blotting and subcellular fractionation by discontinuous centrifugation. We also measured lymphocyte proliferation and natural killer (NK) cell activity.
Results: We confirm robust upregulation of c-Src expression following traumatic stress. c-Src upregulation was accompanied by marked increases in levels of miRNA222; other studied miRNAs were not affected by stress. We also established that PAK1 is a primary target for miRNA222, and that increased levels of miRNA222 following traumatic stress are accompanied by downregulation of PAK1 expression. PAK1 was shown to mediate the association of IL-1RI with lipid rafts and thereby enhance IL-1 signaling. Detailed analyses in cultured neurons and glial cells revealed that PAK1-mediated enhancement of IL-1RI activation is governed to a large extent by c-Src/miRNA222 signaling; this signaling played a central role in the modulation of lymphocyte proliferation and NK cell activity.
Conclusions: Our results suggest that neuroimmune modulation following traumatic stress is mediated by a cascade that involves c-Src-mediated enhancement of miRNA222 expression and downregulation of PAK1, which in turn impairs signaling via IL-1β/IL1-RI, leading to immunosuppression. The regulatory networks involving c-Src/miRNA222 and PAK1/IL-1RI signaling have significant potential for the development of therapeutic approaches designed to promote recovery following traumatic injury.