Upregulation of expression of the close homolog of adhesion molecule L1 (CHL1) by reactive astrocytes in the glial scar reduces axonal regeneration and inhibits functional recovery after spinal cord injury (SCI). Here, we investigate the molecular mechanisms underlying upregulation of CHL1 expression by analyzing the signal transduction pathways in vitro. We show that astrogliosis stimulated by bacterial lipopolysaccharide (LPS) upregulates CHL1 expression in primary cultures of mouse cerebral astrocytes, coinciding with elevated protein synthesis and translocation of protein kinase δ (PKCδ) from cytosol to the membrane fraction. Blocking PKCδ activity pharmacologically and genetically attenuates LPS-induced elevation of CHL1 protein expression through a phosphatidylinositol 3-kinase (PI3K) dependent pathway. LPS induces extracellular signal-regulated kinases (ERK1/2) phosphorylation through PKCδ and blockade of ERK1/2 activation abolishes upregulation of CHL1 expression. LPS-triggered upregulation of CHL1 expression mediated through translocation of nuclear factor κB (NF-κB) to the nucleus is blocked by a specific NF-κB inhibitor and by inhibition of PI3K, PKCδ, and ERK1/2 activities, implicating NF-κB as a downstream target for upregulation of CHL1 expression. Furthermore, the LPSmediated upregulation of CHL1 expression by reactive astrocytes is inhibitory for hippocampal neurite outgrowth in cocultures. Although the LPS-triggered NO-guanylate cyclase-cGMP pathway upregulates glial fibrillary acid protein expression in cultured astrocytes, we did not observe this pathway to mediate LPS-induced upregulation of CHL1 expression. Our results indicate that elevated CHL1 expression by reactive astrocytes requires activation of PI3K/PKCδ-dependent pathways and suggest that reduction of PI3K/PKCδ activity represents a therapeutic target to downregulate CHL1 expression and thus benefit axonal regeneration after SCI.
All Science Journal Classification (ASJC) codes
- Cellular and Molecular Neuroscience
- Close homolog of L1