Repurposing of Tranilast for Potential Neuropathic Pain Treatment by Inhibition of Sepiapterin Reductase in the BH₄ Pathway

Tetrahydrobiopterin (BH4) is a cofactor in the production of various signaling molecules including nitric oxide, dopamine, adrenaline, and noradrenaline. BH4 levels are critical for processes associated with cardiovascular function, inflammation, mood, pain, and neurotransmission. Increasing pieces of evidence suggest that BH4 is upregulated in chronic pain. Sepiapterin reductase (SPR) catalyzes both the reversible reduction of sepiapterin to dihydrobiopterin (BH2) and 6-pyruvoyl-tetrahydrobiopterin to BH4 within the BH4 pathway. Therefore, inhibition of SPR by small molecules can be used to control BH4 production and ultimately alleviate chronic pain. Here, we have used various in silico and in vitro experiments to show that tranilast, licensed for use in bronchial asthma, can inhibit sepiapterin reduction by SPR. Docking and molecular dynamics simulations suggest that tranilast can bind to human SPR (hSPR) at the same site as sepiapterin including S157, one of the catalytic triad residues of hSPR. Colorimetric assays revealed that tranilast was nearly twice as potent as the known hSPR inhibitor, N-acetyl serotonin. Tranilast was able to inhibit hSPR activity both intracellularly and extracellularly in live cells. Triple quad mass spectrophotometry of cell lysates showed a proportional decrease of BH4 in cells treated with tranilast. Our results suggest that tranilast can act as a potent hSPR inhibitor and therefore is a valid candidate for drug repurposing in the treatment of chronic pain

Antenatal architecture and activity of the human heart

We construct the components for a family of computational models of the electrophysiology of the human foetal heart from 60 days gestational age (DGA) to full term. This requires both cell excitation models that reconstruct the myocyte action potentials, and datasets of cardiac geometry and architecture. Fast low-angle shot and diffusion tensor magnetic resonance imaging (DT-MRI) of foetal hearts provides cardiac geometry with voxel resolution of approximately 100 µm. DT-MRI measures the relative diffusion of protons and provides a measure of the average intravoxel myocyte orientation, and the orientation of any higher order orthotropic organization of the tissue. Such orthotropic organization in the adult mammalian heart has been identified with myocardial sheets and cleavage planes between them. During gestation, the architecture of the human ventricular wall changes from being irregular and isotropic at 100 DGA to an anisotropic and orthotropic architecture by 140 DGA, when it has the smooth, approximately 120° transmural change in myocyte orientation that is characteristic of the adult mammalian ventricle. The DT obtained from DT-MRI provides the conductivity tensor that determines the spread of potential within computational models of cardiac tissue electrophysiology. The foetal electrocardiogram (fECG) can be recorded from approximately 60 DGA, and RR, PR and QT intervals between the P, R, Q and T waves of the fECG can be extracted by averaging from approximately 90 DGA. The RR intervals provide a measure of the pacemaker rate, the QT intervals an index of ventricular action potential duration, and its rate-dependence, and so these intervals constrain and inform models of cell electrophysiology. The parameters of models of adult human sinostrial node and ventricular cells that are based on adult cell electrophysiology and tissue molecular mapping have been modified to construct preliminary models of foetal cell electrophysiology, which reproduce these intervals from fECG recordings. The PR and QR intervals provide an index of conduction times, and hence propagation velocities (approx. 1–10 cm s−1, increasing during gestation) and so inform models of tissue electrophysiology. Although the developing foetal heart is small and the cells are weakly coupled, it can support potentially lethal re-entrant arrhythmia