Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) is an intracellular pattern recognition receptor (PRR) that plays an important role in recognizing bacterial pathogens and initiating an immune response. As a PRR, NOD2 recognizes bacterial lipopolysaccharide (LPS), muramyldipeptide (MDP), and other pathogen-associated molecular patterns (PAMPs). NOD2 is a 110 kDa cytoplasmic protein belonging to the Nod-like receptor (NLR) family. Its expression is largely restricted to monocytes and other antigen-presenting cells (APCs). Proteins in the NLR family all contain a nucleotide-binding oligomerization domain (NOD) responsible for coordinating protein oligomerization. NOD2 also contains a C-terminal leucine-rich repeat (LRR) domain with 11 LRR repeats. The LRR domain coordinates ligand binding and other protein-protein interactions. NOD2 also contains two N-terminal caspase associated recruitment domains (CARDs). The CARDs can recruit proteins to promote apoptosis. The NOD2 CARDs are also capable of activating NFκB signaling via a homophilic CARD-CARD interaction with RICK, a serine-threonine kinase (1). RICK then associates with IKKγ to promote IKK-dependent activation of NFκB. Mutations in the NOD2 gene have been associated with inflammatory and autoimmune conditions such as Crohn’s disease, inflammatory bowel disease, Blau syndrome, sarcoidosis, and graft-versus host disease.
When little was known about NOD2 regulation, McDonald et. al. used the NOD2 antibody to discover potential regulators of NOD2 activity (2). The group used the NOD2 antibody (2D9) for co-immunoprecipitation and western blot analysis of possible binding partners. They also used the NOD2 antibody for immunohistochemical studies for colocalization. The group identified Erbin as a negative regulator of NOD2 and confirmed that Erbin binds the CARDs in NOD2. They also determined that Erbin binding to NOD2 prevents NFκB activation via the IKK complex.
Cooney et. al. more recently elucidated a mechanism by which NOD2 signaling induces autophagy in dendritic cells in response to bacterial pathogens (3). First the group showed that MDP stimulation induced autophagy in dendritic cells. Then they used siRNA knockdowns of NOD2, confirmed by western blot with the NOD2 antibody, and measured significantly lower levels of autophagy in the NOD2 knockdowns. The group also used the NOD2 antibody to confirm expression of NOD2 mutants, a model to study the Crohn’s disease polymorphisms. The group identified defects in MHC Class II processing and CD4+ cell responses when NOD2 mutations were present. Interestingly, the group found that polymorphisms in ATG16L1 conferred susceptibility to Crohn’s disease by this same mechanism.
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