Functional expression of NOD2, a novel pattern recognition receptor for bacterial motifs, in primary murine astrocytes

Abstract

There is growing appreciation that resident brain cells can initiate and/or regulate inflammation after trauma or infection in the central nervous system (CNS). Recent studies from our laboratory have begun to shed light on the mechanisms by which astrocytes perceive bacterial challenges by demonstrating the functional expression of Toll-like receptors (TLR) in this cell type. In the present study, we demonstrate that astrocytes also express members of the novel nucleotide-binding oligomerization domain (NOD) family of proteins that can serve as cytosolic pattern recognition receptors. We show that isolated cultures of murine astrocytes constitutively express robust levels of NOD2, a molecule that can recognize a minimal peptidoglycan motif. Expression of NOD2 is significantly upregulated after exposure to two disparate and clinically relevant bacterial pathogens of the CNS, Borrelia burgdorferi and Neisseria meningitidis. Similarly, NOD2 protein expression is elevated after exposure to specific bacterial ligands for TLRs. Importantly, we show that astrocytes express Rip2 kinase, an essential downstream effector molecule for NOD-mediated cell responses, and demonstrate that this expression is upregulated after bacterial challenge. Furthermore, we confirm the functional nature of NOD2 in astrocytes by demonstrating that a specific ligand for this receptor induces significant inflammatory cytokine production and augments immune responses induced by TLR ligation. Taken together, the present demonstration that astrocytes express functional NOD2 proteins may represent a potentially important mechanism by which this glial cell type initiates either protective host responses within the brain or the progression of damaging CNS inflammation.