Turning the tide on Alzheimer’s disease: modulation of γ-secretase

Alzheimer’s disease (AD) is the most common type of neurodegenerative disorder. Amyloid-beta (Aβ) plaques are integral to the “amyloid hypothesis,” which states that the accumulation of Aβ peptides triggers a cascade of pathological events leading to neurodegeneration and ultimately AD. While the FDA approved aducanumab, the first Aβ-targeted therapy, multiple safe and effective treatments will be needed to target the complex pathologies of AD. γ-Secretase is an intramembrane aspartyl protease that is critical for the generation of Aβ peptides. Activity and specificity of γ-secretase are regulated by both obligatory subunits and modulatory proteins. Due to its complex structure and function and early clinical failures with pan inhibitors, γ-secretase has been a challenging drug target for AD. γ-secretase modulators, however, have dramatically shifted the approach to targeting γ-secretase. Here we review γ-secretase and small molecule modulators, from the initial characterization of a subset of NSAIDs to the most recent clinical candidates. We also discuss the chemical biology of γ-secretase, in which small molecule probes enabled structural and functional insights into γ-secretase before the emergence of high-resolution structural studies. Finally, we discuss the recent crystal structures of γ-secretase, which have provided valuable perspectives on substrate recognition and molecular mechanisms of small molecules. We conclude that modulation of γ-secretase will be part of a new wave of AD therapeutics.

Novel enhancers of guanylyl cyclase-A activity via allosteric modulation

Natriuretic peptide receptor (NPR)-A (also known as NPR-A, NPR1 or guanylyl cyclase-A, GC-A) is an attractive but challenging target to activate with small molecules. GC-A is activated by endogenous atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), and this activation leads to the production of cyclic guanosine monophosphate (cGMP). This system plays an important role in the regulation of cardiovascular and renal homeostasis. However, utilization of this receptor as a drug target has so far been limited to peptides, even though small molecule modulators allow oral administration and longer half-life. We have identified small molecular allosteric enhancers of GC-A, which strengthened ANP or BNP activation in various in vitro and ex vivo systems. These compounds do not mediate their actions through previously described allosteric binding sites or via known mechanisms of action. In addition, their selectivity and activity are dependent on only one amino acid in GC-A. Our findings show that there is a novel allosteric binding site on GC-A, which can be targeted by small molecules that increase the signaling effects of ANP and BNP.

Discovery of small-molecule modulators of melanogenesis by docking-based virtual screening

Background: Vitiligo is a relatively common depigmenting skin disorder. UV light stimulation is often used to obtain repigmentation. Wnt signaling regulates melanocyte differentiation, and expression of TYR is upregulated in narrow-band UVB-treated epidermis. Manipulation of these two pathways by drugs could serve as one of the therapeutic approaches for durable repigmentation. Methods and results: CD9 was identified as a novel TYR activator by virtual screening and bioactivity assay. CD9 activated the Wnt signaling pathway through triggering translocation of β-catenin from cytoplasm to nucleus. Conclusion: The pathogenesis of vitiligo is complicated and varies with each individual, so combination therapy may be much more suitable for treatment of vitiligo. CD9 could synergize with other anti-inflammatory compounds or autoimmune suppressors to shorten repigmentation time and improve efficacy.

Molecular dynamics simulations reveal the selectivity mechanism of structurally similar agonists to TLR7 and TLR8

TLR7 and TLR8 are key members of the Toll-like receptor family, playing crucial roles in the signaling pathways of innate immunity, and thus become attractive therapeutic targets of many diseases including infections and cancer. Although TLR7 and TLR8 show a highly degree of sequence homology, their biological response to small molecule binding is very different. Aiming to understand the mechanism of selective profiles of small molecule modulators against TLR7 and TLR8, we carried out molecular dynamic simulations on three imidazoquinoline derivatives bound to the receptors separately. They are Resiquimod (R), Hybrid-2 (H), and Gardiquimod (G), selective agonists of TLR7 and TLR8. Our MD trajectories indicated that in the complex of TLR7-R and TLR7-G, the two chains forming the TLR7 dimer tended to remain “open” conformation, while the rest systems maintained in the closed format. The agonists R, H, and G developed conformational deviation mainly on the aliphatic tail. Furthermore, we attempted to quantify the selectivity between TLR7 and TLR8 by binding free energies via MM-GBSA method. It showed that the three selected modulators were more favorable for TLR7 than TLR8, and the ranking from the strongest to the weakest was H, R and G, aligning well with experiment data. In the TLR7, the flexible and hydrophobic aliphatic side chain of H has stronger van der Waals interactions with Val381 and Phe351 but only pick up interaction with one amino acid residue i.e. Tyr353 of TLR8. Unsurprisingly, the positively charged side chain of G has less favor interaction with Ile585 of TLR7 and Val573 of TLR8 explaining G is weak agonist in both TLR7 and TLR8. All three imidazoquinolines can form stable hydrogen bonds with Asp555 of TLR7 and the corresponding Asp543 of TLR8. In brief, the set of total 400ns MD studies sheds light on the potential selective mechanisms of agonists towards TLR7 and TLR8, indicating the van der Waals interaction as the driving force for the agonists binding, thus provides us insights for more potent and selective modulators to cooperate with the hydrophobic nature of the binding pocket.

Chloride transport modulators as drug candidates

Chloride transport across cell membranes is broadly involved in epithelial fluid transport, cell volume and pH regulation, muscle contraction, membrane excitability, and organellar acidification. The human genome encodes at least 53 chloride transporting proteins with expression in cell plasma or intracellular membranes, which include chloride channels, exchangers and cotransporters, some having broad anion specificity. Loss of function mutations in chloride transporters cause a wide variety of human diseases, including cystic fibrosis, secretory diarrhea, kidney stones, salt wasting nephropathy, myotonia, osteopetrosis, hearing loss and goiter. While impactful advances have been made in the past decade in drug treatment of cystic fibrosis using small molecule modulators of the defective cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, other chloride channels and solute carrier proteins (SLCs) represent relatively underexplored target classes for drug discovery. New opportunities have emerged for development of chloride transport modulators as potential therapeutics for secretory diarrheas, constipation, dry eye disorders, kidney stones, polycystic kidney disease, hypertension and osteoporosis. Approaches to chloride transport-targeted drug discovery are reviewed herein, with focus on chloride channel and exchanger classes in which recent preclinical advances have been made in the identification of small molecule modulators and in proof of concept testing in experimental animal models.

Elexacaftor is a CFTR potentiator and acts synergistically with ivacaftor during acute and chronic treatment

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), which lead to early death due to progressive lung disease. The development of small-molecule modulators that directly interact with CFTR to aid in protein folding (“correctors”) and/or increase channel function (“potentiators”) have proven to be highly effective in the therapeutic treatment of CF. Notably, incorporation of the next-generation CFTR corrector, elexacaftor, into a triple combination therapeutic (marketed as Trikafta) has shown tremendous clinical promise in treating CF caused by F508del-CFTR. Here, we report on a newly-described role of elexacaftor as a CFTR potentiator. We explore the acute and chronic actions, pharmacology, and efficacy of elexacaftor as a CFTR potentiator in restoring function to multiple classes of CFTR mutations. We demonstrate that the potentiating action of elexacaftor exhibits multiplicative synergy with the established CFTR potentiator ivacaftor in rescuing multiple CFTR class defects, indicating that a new combination therapeutic of ivacaftor and elexacaftor could have broad impact on CF therapies.

Novel c-Jun N-Terminal Kinase (JNK) Inhibitors with an 11H-Indeno[1,2-b]quinoxalin-11-one Scaffold

c-Jun N-terminal kinase (JNK) plays a central role in stress signaling pathways implicated in important pathological processes, including rheumatoid arthritis and ischemia-reperfusion injury. Therefore, inhibition of JNK is of interest for molecular targeted therapy to treat various diseases. We synthesized 13 derivatives of our reported JNK inhibitor 11H-indeno[1,2-b]quinoxalin-11-one oxime and evaluated their binding to the three JNK isoforms and their biological effects. Eight compounds exhibited submicromolar binding affinity for at least one JNK isoform. Most of these compounds also inhibited lipopolysaccharide (LPS)-induced nuclear factor-κB/activating protein 1 (NF-κB/AP-1) activation and interleukin-6 (IL-6) production in human monocytic THP1-Blue cells and human MonoMac-6 cells, respectively. Selected compounds (4f and 4m) also inhibited LPS-induced c-Jun phosphorylation in MonoMac-6 cells, directly confirming JNK inhibition. We conclude that indenoquinoxaline-based oximes can serve as specific small-molecule modulators for mechanistic studies of JNKs, as well as potential leads for the development of anti-inflammatory drugs.