A novel gain-of-function phosphorylation site modulates PTPN22 inhibition of TCR signaling

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• Protein tyrosine phosphatase nonreceptor type 22 (PTPN22) is a key regulator of T cell receptor (TCR) signaling and a target for cancer immunotherapy.
• The posttranslational regulation of PTPN22, particularly its phosphorylation, remains largely uncharacterized.

Purpose of the Study:

• To identify and characterize a novel phosphorylation site on PTPN22 (Ser325) and elucidate its role in regulating PTPN22 function.
• To investigate the impact of Ser325 phosphorylation on TCR signaling, downstream transcriptional activity, and protein interactions.

Main Methods:

• Phosphorylation site identification using phospho-mass spectrometry.
• Functional characterization using CRISPR/Cas9 gene editing and site-directed mutagenesis (glutamic acid substitution).
• Analysis of downstream transcriptional activity and gene expression using next-generation sequencing.
• Structural and mechanistic investigations employing deuterium exchange mass spectrometry and NMR spectroscopy.

Main Results:

• Ser325 phosphorylation, mediated by glycogen synthase kinase-3 (GSK3) upon TCR stimulation, enhances PTPN22's TCR-inhibitory activity.
• Suppression of Ser325 phosphorylation by CRISPR/Cas9 enhanced TCR signaling, while mimicking phosphorylation inhibited it.
• Ser325 phosphorylation alters downstream transcriptional activity by influencing the binding of specific protein complexes and differentially regulates chemokine and T cell activation pathways.
• In vitro kinetic data indicate context-dependent modulation of PTPN22 activity.

Conclusions:

• The study identifies and functionally characterizes Ser325 phosphorylation as a critical regulatory mechanism for PTPN22.
• This novel regulatory site significantly impacts TCR signaling, offering potential therapeutic avenues for autoimmune diseases and cancer immunotherapy.
• Findings provide mechanistic insights that could guide the development of allosteric modulators targeting PTPN22.