3 research outputs found
Peptidoglycan Modifications Tune the Stability and Function of the Innate Immune Receptor Nod2
Natural
modifications of peptidoglycan modulate the innate immune
response. Peptidoglycan derivatives activate this response via the
intracellular innate immune receptor, Nod2. To probe how these modifications
alter the response, a novel and efficient carbohydrate synthesis was
developed to allow for late-stage modification of the amine at the
2-position. Modification of the carbohydrate was found to be important
for stabilizing Nod2 and generating the proper response. The native
Nod2 ligands demonstrate a significant increase in the cellular stability
of Nod2. Moreover, changing the identity of the natural ligands at
the carbohydrate 2-position allows for the Nod2-dependent immune response
to be either up-regulated or down-regulated. The ligand structure
can be adjusted to tune the Nod2 response, suggesting that other innate
immune receptors and their ligands could use a similar strategy
Membrane Association Dictates Ligand Specificity for the Innate Immune Receptor NOD2
The
human gut must regulate its immune response to resident and
pathogenic bacteria, numbering in the trillions. The peptidoglycan
component of the bacterial cell wall is a dense and rigid structure
that consists of polymeric carbohydrates and highly cross-linked peptides
which offers protection from the host and surrounding environment.
Nucleotide-binding oligomerization domain-containing protein 2 (NOD2),
a human membrane-associated innate immune receptor found in the gut
epithelium and mutated in an estimated 30% of Crohn’s disease
patients, binds to peptidoglycan fragments and initiates an immune
response. Using a combination of chemical synthesis, advanced analytical
assays, and protein biochemistry, we tested the binding of a variety
of synthetic peptidoglycan fragments to wild-type (WT)-NOD2. Only
when the protein was presented in the native membrane did binding
measurements correlate with a NOD2-dependent nuclear factor kappa-light-chain-enhancer
of activated B cells (NF-κB) response, supporting the hypothesis
that the native-membrane environment confers ligand specificity to
the NOD2 receptor for NF-κB signaling. While <i>N</i>-acetyl-muramyl dipeptide (MDP) has been thought to be the minimal
peptidoglycan fragment necessary to activate a NOD2-dependent immune
response, we found that fragments with and without the dipeptide moiety
are capable of binding <i>and</i> activating a NOD2-dependent
NF-κB response, suggesting that the carbohydrate moiety of the
peptidoglycan fragments is the minimal functional epitope. This work
highlights the necessity of studying NOD2-ligand binding in systems
that resemble the receptor’s natural environment, as the cellular
membrane and/or NOD2 interacting partners appear to play a crucial
role in ligand binding and in triggering an innate immune response
Crohn’s Disease Variants of Nod2 Are Stabilized by the Critical Contact Region of Hsp70
Nod2
is a cytosolic, innate immune receptor responsible for binding
to bacterial cell wall fragments such as muramyl dipeptide (MDP).
Upon binding, subsequent downstream activation of the NF-κB
pathway leads to an immune response. Nod2 mutations are correlated
with an increased susceptibility to Crohn’s disease (CD) and
ultimately result in a misregulated immune response. Previous work
had demonstrated that Nod2 interacts with and is stabilized by the
molecular chaperone Hsp70. In this work, it is shown using purified
protein and <i>in vitro</i> biochemical assays that the
critical Nod2 CD mutations (G908R, R702W, and 1007fs) preserve the
ability to bind bacterial ligands. A limited proteolysis assay and
luciferase reporter assay reveal regions of Hsp70 that are capable
of stabilizing Nod2 and rescuing CD mutant activity. A minimal 71-amino
acid subset of Hsp70 that stabilizes the CD-associated variants of
Nod2 and restores a proper immune response upon activation with MDP
was identified. This work suggests that CD-associated Nod2 variants
could be stabilized <i>in vivo</i> with a molecular chaperone