Membrane Interaction and Protein Kinase C-C1 Domain Binding Properties of 4-Hydroxy-3-(hydroxymethyl) Phenyl Ester Analogues

Targeting PKC in microglia to promote remyelination and repair in the CNS

Microglia and CNS-infiltrating macrophages play significant roles in the pathogenesis of neuroinflammatory and neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and multiple sclerosis. Prolonged and dysregulated inflammatory responses by these innate immune cells can have deleterious effects on the surrounding CNS microenvironment, which can worsen neurodegeneration and demyelination. However, although chronic activation of pro-inflammatory microglia is maladaptive, other functional microglial subtypes play beneficial roles during CNS repair and regeneration. Therefore, there is a tremendous interest in understanding the underlying mechanism of the activation of these reparative/regenerative microglia. In this review, we focus on the potential role of PKC, a downstream signaling molecule of TREM2 and PLCγ2, and PKC modulators in promoting the activation of reparative/regenerative microglial subtypes as a novel therapy for neuroinflammatory and neurodegenerative diseases.

From fluorogens to fluorophores by elucidation and suppression of ultrafast excited state processes of a Schiff base

Strategies have been explored for developing strongly fluorescent species out of a weakly fluorescent Schiff base, 2-(((pyridin-2-ylmethyl)imino)methyl)phenol (salampy). The locally excited enolic state of salampy undergoes an intramolecular proton transfer with a time constant of ca. 200 fs. The emissive cis keto state thus formed decays completely within 50 ps. Its fast decay and miniscule fluorescence quantum yield are attributed to efficient non-radiative channels associated with conformational relaxation. The anionic form, salampy^-, has a significantly longer fluorescence lifetime of 800 ps. Its emissive state evolves in tens of picoseconds, from the locally excited state, by solvent and conformational relaxation. Both the neutral and anionic forms have a fluorescence lifetime of about 6 ns at 77 K, a temperature at which all activated nonradiative channels are blocked. This lifetime is similar to that obtained at room temperature, upon rigidification of the anion by complexation with Zn²⁺. Two such complexes have been studied. The first is binuclear, with acetate bridge between the two Zn²⁺ ions. The second, with ClO₄^- as the counterion, is mononuclear with two salampy ligands ligating the metal ion. Unlike a previous report on a different Schiff base, in which the ligands are π-stacked in its dimeric Zn²⁺ complex, no additional nonradiative deactivation pathway opens up in the Zn complexes of salampy, which are devoid of such stacking. The complex of salampy with Al³⁺ has an even longer fluorescence lifetime of 9 ns, indicating a greater degree of rigidification and consequent suppression of nonradiative processes.

4,5-Disubstituted 1,2,3-triazoles: Effective Inhibition of Indoleamine 2,3-Dioxygenase 1 Enzyme Regulates T cell Activity and Mitigates Tumor Growth

The improvement of body's own immune system is considered one of the safest approaches to fight against cancer and several other diseases. Excessive catabolism of the essential amino acid, L-tryptophan (L-Trp) assists the cancer cells to escape normal immune obliteration. The formation of disproportionate kynurenine and other downstream metabolites suppress the T cell functions. Blocking of this immunosuppressive mechanism is considered as a promising approach against cancer, neurological disorders, autoimmunity, and other immune-mediated diseases. Overexpression of indoleamine 2,3-dioxygenase 1 (IDO1) enzyme is directly related to the induction of immunosuppressive mechanisms and represents an important therapeutic target. Several classes of small molecule-based IDO1 inhibitors have been already reported, but only few compounds are currently being evaluated in various stages of clinical trials as adjuvants or in combination with chemo- and radiotherapies. In the quest for novel structural class(s) of IDO1 inhibitors, we developed a series of 4,5-disubstituted 1,2,3-triazole derivatives. The optimization of 4,5-disubstituted 1,2,3-triazole scaffold and comprehensive biochemical and biophysical studies led to the identification of compounds, 3i, 4i, and 4k as potent and selective inhibitors of IDO1 enzyme with IC₅₀ values at a low nanomolar level. These potent compounds also showed strong IDO1 inhibitory activities in MDA-MB-231 cells with no/negligible level of cytotoxicity. The T cell activity studies revealed that controlled regulation of IDO1 enzyme activity in the presence of these potent compounds could induce immune response against breast cancer cells. The compounds also showed excellent in vivo antitumor efficacy (of tumor growth inhibition = 79-96%) in the female Swiss albino mice. As a consequence, this study describes the first example of 4,5-disubstituted 1,2,3-triazole based IDO1 inhibitors with potential applications for immunotherapeutic studies.

Synthesis and evaluation of oxindoles as promising inhibitors of the immunosuppressive enzyme indoleamine 2,3-dioxygenase 1

Indoleamine 2,3-dioxygenase 1 (IDO1) is considered as an important therapeutic target for the treatment of cancer, chronic infections and other diseases that are associated with immune suppression. Recent developments in understanding the catalytic mechanism of the IDO1 enzyme revealed that conversion of l-tryptophan (l-Trp) to N-formylkynurenine proceeded through an epoxide intermediate state. Accordingly, we synthesized a series of 3-substituted oxindoles from l-Trp, tryptamine and isatin. Compounds with C3-substituted oxindole moieties showed moderate inhibitory activity against the purified human IDO1 enzyme. Their optimization led to the identification of potent compounds, 6, 22, 23 and 25 (IC₅₀ = 0.19 to 0.62 μM), which are competitive inhibitors of IDO1 with respect to l-Trp. These potent compounds also showed IDO1 inhibition potencies in the low-micromolar range (IC₅₀ = 0.33-0.49 μM) in MDA-MB-231 cells. The cytotoxicity of these potent compounds was trivial in different model cancer (MDA-MB-231, A549 and HeLa) cells and macrophage (J774A.1) cells. Stronger selectivity for the IDO1 enzyme (124 to 210-fold) over the tryptophan 2,3-dioxygenase (TDO) enzyme was also observed for these compounds. These results suggest that the oxindole moiety of the compounds could mimic the epoxide intermediate state of l-Trp. Therefore, the structural simplicity and low-micromolar inhibition potencies of these 3-substituted oxindoles make them quite attractive for further investigation of IDO1 function and immunotherapeutic applications.

Insights into the inhibitory mechanism of triazole-based small molecules on phosphatidylinositol-4,5-bisphosphate binding pleckstrin homology domain

Background

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] is an important regulator of several cellular processes and a precursor for other second messengers which are involved in cell signaling pathways. Signaling proteins preferably interact with PI(4,5)P₂ through its pleckstrin homology (PH) domain. Efforts are underway to design small molecule-based antagonist, which can specifically inhibit the PI(4,5)P₂/PH-domain interaction to establish an alternate strategy for the development of drug(s) for phosphoinositide signaling pathways.

Methods

Surface plasmon resonance, molecular docking, circular dichroism, competitive Förster resonance energy transfer, isothermal titration calorimetric analyses and liposome pull down assay were used.

Results

In this study, we employed 1,2,3-triazol-4-yl methanol containing small molecule (CIPs) as antagonists for PI(4,5)P₂/PH-domain interaction and determined their inhibitory effect by using competitive-surface plasmon resonance analysis (IC₅₀ ranges from 53 to 159 nM for PI(4,5)P₂/PLCδ1-PH domain binding assay). We also used phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P₃], phosphatidylinositol 3,4-bisphosphate [PI(3,4)P₂], PI(4,5)P₂ specific PH-domains to determine binding selectivity of the compounds. Various physicochemical analyses showed that the compounds have weak affect on fluidity of the model membrane but, strongly interact with the phospholipase C δ1 (PLCδ1)-PH domains. The 1,2,3-triazol-4-yl methanol moiety and nitro group of the compounds are essential for their exothermic interaction with the PH-domains. Potent compound can efficiently displace PLCδ1-PH domain from plasma membrane to cytosol in A549 cells.

Conclusions

Overall, our studies demonstrate that these compounds interact with the PIP-binding PH-domains and inhibit their membrane recruitment.

General significance

These results suggest specific but differential binding of these compounds to the PLCδ1-PH domain and emphasize the role of their structural differences in binding parameters. These triazole-based compounds could be directly used/further developed as potential inhibitor for PH domain-dependent enzyme activity.

Elucidating the interaction of γ-hydroxymethyl-γ-butyrolactone substituents with model membranes and protein kinase C-C1 domains

The protein kinase C (PKC) family of proteins is an attractive drug target. Dysregulation of PKC-dependent signalling pathways is related to several human diseases like cancer, immunological and other diseases. We approached the problem of altering PKC activities by developing C1 domain-based PKC ligands. In this report γ-hydroxymethyl-γ-butyrolactone (HGL) substituents were investigated in an effort to develop small molecule-based PKC regulators with higher specificity for C1 domain than the endogenous diacylglycerols (DAGs). Extensive analysis of membrane-ligands interaction measurements revealed that the membrane-active compounds strongly interact with the lipid bilayers and the hydrophilic parts of compounds localize at the bilayer/water interface. The pharmacophores like hydroxymethyl, carbonyl groups and acyl-chain length of the compounds are crucial for their interaction with the C1 domain proteins. The potent compounds showed more than 17-fold stronger binding affinity for the C1 domains than DAG under similar experimental conditions. Nonradioactive kinase assay confirmed that these potent compounds have similar or better PKC dependent phosphorylation capabilities than DAG under similar experimental conditions. Hence, our findings reveal that these HGL analogues represent an attractive group of structurally simple C1 domain ligands that can be further structurally altered to improve their potencies.

Inhibition of phosphatidylinositol-3,4,5-trisphosphate binding to the AKT pleckstrin homology domain by 4-amino-1,2,5-oxadiazole derivatives

4-Amino-1,2,5-oxadiazole derivatives has been developed as an inhibitor of AKT pleckstrin homology domain.

Synthesis and protein kinase C (PKC)-C1 domain binding properties of diacyltetrol based anionic lipids

Protein kinase C-C1 domain binding specificity of the anionic hybrid lipids.