Development of Novel Anti-Leishmanials: The Case for Structure-Based Approaches

The neglected tropical disease (NTD) leishmaniasis is the collective name given to a diverse group of illnesses caused by ~20 species belonging to the genus Leishmania, a majority of which are vector borne and associated with complex life cycles that cause immense health, social, and economic burdens locally, but individually are not a major global health priority. Therapeutic approaches against leishmaniasis have various inadequacies including drug resistance and a lack of effective control and eradication of the disease spread. Therefore, the development of a rationale-driven, target based approaches towards novel therapeutics against leishmaniasis is an emergent need. The utilization of Artificial Intelligence/Machine Learning methods, which have made significant advances in drug discovery applications, would benefit the discovery process. In this review, following a summary of the disease epidemiology and available therapies, we consider three important leishmanial metabolic pathways that can be attractive targets for a structure-based drug discovery approach towards the development of novel anti-leishmanials. The folate biosynthesis pathway is critical, as Leishmania is auxotrophic for folates that are essential in many metabolic pathways. Leishmania can not synthesize purines de novo, and salvage them from the host, making the purine salvage pathway an attractive target for novel therapeutics. Leishmania also possesses an organelle glycosome, evolutionarily related to peroxisomes of higher eukaryotes, which is essential for the survival of the parasite. Research towards therapeutics is underway against enzymes from the first two pathways, while the third is as yet unexplored.

The coiled-coil domain of glycosomal membrane-associated Leishmania donovani PEX14: cloning, overexpression, purification and preliminary crystallographic analysis

The glycosomal membrane-associated Leishmania donovani protein PEX14, which plays a crucial role in protein import from the cytosol to the glycosomal matrix, consists of three domains: an N-terminal domain where the signalling molecule binds, a transmembrane domain and an 84-residue coiled-coil domain (CC) that is responsible for oligomerization. CCs are versatile domains that participate in a variety of functions including supramolecular assembly, cellular signalling and transport. Recombinant PEX14 CC was cloned, overexpressed, affinity-purified with in-column thrombin cleavage and further purified by size-exclusion chromatography. Crystals that diffracted to 1.98 Å resolution were obtained from a condition consisting of 1.4 M sodium citrate tribasic dihydrate, 0.1 M HEPES buffer pH 7.5. The crystals belonged to the monoclinic space group C2, with unit-cell parameters a = 143.98, b = 32.62, c = 95.62 Å, β = 94.68°. Structure determination and characterization are in progress.

Acylcarnitines at the Membrane Surface: Insertion Parameters for a Mitochondrial Leaflet Model

Excessive accumulation of acylcarnitines (ACs), often caused by metabolic disorders, has been associated with obesity, arrhythmias, cardiac ischemia, insulin resistance, etc. Mechanisms whereby elevated ACs might contribute to pathophysiological effects remain largely unexplored. We have aimed to gain insight into AC interactions with the mitochondrial inner membrane. To model its outer leaflet, Langmuir monolayers and cushioned supported bilayers were employed. Their interactions with ACs were monitored with epifluorescence microscopy, which revealed a local leaflet expansion upon exposure to elevated concentrations of a long-chain AC, plausibly caused by its insertion. To assess the AC insertion parameters, constant-pressure insertion assays were performed. A value of 21 ± 3 Å² was obtained for the AC insertion area, which is roughly the same as the cross-sectional area of an acyl chain. By contrast, the carnitine moiety was found to require an area of 37 ± 3 Å². The AC insertion has thus been concluded to involve solely the AC acyl chain. This mode of insertion implies that the carnitine moiety, with its nontitratable positive charge, is left dangling at the membrane surface, which is likely to alter the surface electrostatics of the outer leaflet. The extrapolation of these findings has enabled us to hypothesize that, by altering the morphology and surface electrostatics of the outer leaflet, the insertion of ACs, in particular their long-chain counterparts, may trigger a nonspecific activation of signaling pathways in the inner mitochondrial membrane, thereby modulating its function and potentially leading to pathophysiological responses.

Membrane topologies of PEX13 and PEX14 provide new insights on the mechanism of protein import into peroxisomes

PEX13 and PEX14 are two core components of the so-called peroxisomal docking/translocation module, the transmembrane hydrophilic channel through which newly synthesized peroxisomal proteins are translocated into the organelle matrix. The two proteins interact with each other and with PEX5, the peroxisomal matrix protein shuttling receptor, through relatively well characterized domains. However, the topologies of these membrane proteins are still poorly defined. Here, we subjected proteoliposomes containing PEX13 or PEX14 and purified rat liver peroxisomes to protease-protection assays and analyzed the protected protein fragments by mass spectrometry, Edman degradation and western blotting using antibodies directed to specific domains of the proteins. Our results indicate that PEX14 is a bona fide intrinsic membrane protein with a N_in -C_out topology, and that PEX13 adopts a N_out -C_in topology, thus exposing its carboxy-terminal Src homology 3 [SH3] domain into the organelle matrix. These results reconcile several enigmatic findings previously reported on PEX13 and PEX14 and provide new insights into the organization of the peroxisomal protein import machinery. ENZYMES: Trypsin, EC3.4.21.4; Proteinase K, EC3.4.21.64; Tobacco etch virus protease, EC3.4.22.44.