Fig 2: Validation of nilotinib as a p38/MK2 PPI Inhibitor. (A) Thermal shift assay (TSA) showing dose-dependent stabilization of recombinant His-tagged p38 by nilotinib (ΔT max = 8.22 °C), consistent with direct binding. (B) TSA profile for SR318, a type II ATP-competitive p38 inhibitor, used as a positive control (ΔT max = 13.47 °C). (C) Nilotinib competes with His-MK2 346–400 fragment for VF-p38 in a cell lysate-based TR-FRET assay. (D) Quantitative qRT-PCR analysis showing that nilotinib significantly (p-value <0.05) suppresses LPS-induced TNF-α, IL-6, and IL-1β expression in HMC3 microglial cells. P38 inhibitors SR318 and VX-745 were used as positive controls. (E) Nilotinib disrupts the endogenous p38/MK2 complex in HMC3 cells, as shown by coimmunoprecipitation, correlating with cytokine suppression. (F) qRT-PCR analysis showing that nilotinib suppresses LPS/IFNγ-induced TNF-α expression in the human iPSC-derived microglia (iMGL). (G) TR-FRET assay with recombinant p38 and MK2 proteins purified from E. coli demonstrated direct inhibition of the complex by nilotinib (IC50 = 2.2 μM). In contrast, ATP-site inhibitors VX-745 and SR318 failed to disrupt the interaction, supporting a non-ATP-competitive mechanism for nilotinib activity. (H) Nilotinib demonstrates a weak inhibition of p38/ATF2 PPI (IC50 > 30 μM, maximal inhibition ∼ 37%) in a TR-FRET assay with recombinant purified His-p38 and GST-ATF2. The inhibition of His-p38/GST-MK2 PPI by nilotinib was monitored in parallel.

Fig 3: Structure-guided mapping and peptide validation of the p38/MK2 interaction interface. (A) The cocrystal structure of the p38/MK2 complex (PDB ID: 6TCA). The molecular surface of p38 is shown in gray. The p38 docking groove is highlighted in yellow. MK2 is shown as green ribbons. (B) The MK2 D345-H400 docking motif bound to the p38 docking groove is colored based on its fragments tested in this study: D345-H400 is colored in green, I370–L393 in blue, and I370-L382 in red. (C) The binding curve from a fluorescence polarization assay showing high-affinity binding of FITC-labeled MK2 370–393 peptide to His-tagged p38 (EC50 = 26.9 nM). (D) Dose–response curves from TR-FRET inhibition assays demonstrating that both MK2 370–393 and 369–382 peptides disrupt the p38/MK2 complex (IC50 = 0.42 μM and 4.26 μM, respectively).

Fig 4: α 1 -Adrenergic antagonists disrupt the p38/MK2 interface and suppress cytokine production in microglial cells. (A) Chemical structures of doxazosin, terazosin, and alfuzosin, three α1-adrenergic receptor antagonists identified from the high-throughput screen. (B) Dose–response TR-FRET assays using recombinant purified p38 and MK2 proteins demonstrate that all three compounds inhibit the p38/MK2 protein–protein interaction, with IC50 values of 4.4 μM (doxazosin), 6.2 μM (terazosin), and 6.9 μM (alfuzosin). (C) The compound activity was confirmed in a cell lysate-based TR-FRET format, showing a moderate reduction in potency relative to the recombinant protein assay. (D) In a complementary TR-FRET assay using HEK293T lysates coexpressing VF-tagged p8 and a His-tagged MK2 346–400 docking peptide, all three α1-antagonists and nilotinib dose-dependently disrupted peptide binding to p38, consistent with direct competition at the docking interface. (E) qRT-PCR analysis in HMC3 microglial cells shows that all three compounds significantly (p-values <0.05) suppressed LPS-induced expression of TNF-α, IL-6, and IL-1β, similarly to known p38 inhibitors SR318 and VX745, demonstrating effective functional inhibition of p38/MK2 signaling in a disease-relevant context.

Fig 5: Chemical structures of nilotinib and ten analogs evaluated for p38/MK2 PPI inhibition using a TR-FRET assay with recombinant purified proteins. IC50 values are shown for compounds exhibiting measurable activity; compounds with less than 50% inhibition at 30 μM are indicated as not determined (N.D.).