Identification of Chalcone Derivatives as Inhibitors of Leishmania infantum Arginase and Promising Antileishmanial Agents

Arginase catalyzes the hydrolysis of l-arginine into l-ornithine and urea, acting as a key enzyme in the biosynthesis of polyamines. Leishmania growth and survival is dependent on polyamine biosynthesis; therefore, inhibition of Leishmania arginase may be a promising therapeutic strategy. Here, we evaluated a series of thirty-six chalcone derivatives as potential inhibitors of Leishmania infantum arginase (LiARG). In addition, the activity of selected inhibitors against L. infantum parasites was assessed in vitro. Seven compounds exhibited LiARG inhibition above 50% at 100 μM. Among them, compounds LC41, LC39, and LC32 displayed the greatest inhibition values (72.3 ± 0.3%, 71.9 ± 11.6%, and 69.5 ± 7.9%, respectively). Molecular docking studies predicted hydrogen bonds and hydrophobic interactions between the most active chalcones (LC32, LC39, and LC41) and specific residues from LiARG's active site, such as His140, Asn153, His155, and Ala193. Compound LC32 showed the highest activity against L. infantum promastigotes (IC₅₀ of 74.1 ± 10.0 μM), whereas compounds LC39 and LC41 displayed the best results against intracellular amastigotes (IC₅₀ of 55.2 ± 3.8 and 70.4 ± 9.6 μM, respectively). Moreover, compound LC39 showed more selectivity against parasites than host cells (macrophages), with a selectivity index (SI) of 107.1, even greater than that of the reference drug Fungizone®. Computational pharmacokinetic and toxicological evaluations showed high oral bioavailability and low toxicity for the most active compounds. The results presented here support the use of substituted chalcone skeletons as promising LiARG inhibitors and antileishmanial drug candidates.

Leishmania infantum arginase: biochemical characterization and inhibition by naturally occurring phenolic substances

Inhibition of Leishmania arginase leads to a decrease in parasite growth and infectivity and thus represents an attractive therapeutic strategy. We evaluated the inhibitory potential of selected naturally occurring phenolic substances on Leishmania infantum arginase (ARGLi) and investigated their antileishmanial activity in vivo. ARGLi exhibited a V_max of 0.28 ± 0.016 mM/min and a K_m of 5.1 ± 1.1 mM for L-arginine. The phenylpropanoids rosmarinic acid and caffeic acid (100 µM) showed percentages of inhibition of 71.48 ± 0.85% and 56.98 ± 5.51%, respectively. Moreover, rosmarinic acid and caffeic acid displayed the greatest effects against L. infantum with IC₅₀ values of 57.3 ± 2.65 and 60.8 ± 11 μM for promastigotes, and 7.9 ± 1.7 and 21.9 ± 5.0 µM for intracellular amastigotes, respectively. Only caffeic acid significantly increased nitric oxide production by infected macrophages. Altogether, our results broaden the current spectrum of known arginase inhibitors and revealed promising drug candidates for the therapy of visceral leishmaniasis.

Yolk hydrolases in the eggs of Anticarsia gemmatalis hubner (Lepidoptera: Noctuidae): a role for inorganic polyphosphate towards yolk mobilization

Despite being the main insect pest on soybean crops in the Americas, very few studies have approached the general biology of the lepidopteran Anticarsia gemmatalis and there is a paucity of studies with embryo formation and yolk mobilization in this species. In the present work, we identified an acid phosphatase activity in the eggs of A. gemmatalis (agAP) that we further characterized by means of biochemistry and cell biology experiments. By testing several candidate substrates, this enzyme proved chiefly active with phosphotyrosine; in vitro assays suggested a link between agAP activity and dephosphorylation of egg yolk phosphotyrosine. We also detected strong activity with endogenous and exogenous short chain polyphosphates (PolyP), which are polymers of phosphate residues involved in a number of physiological processes. Both agAP activity and PolyP were shown to initially concentrate in small vesicles clearly distinct from typically larger yolk granules, suggesting subcellular compartmentalization. As PolyP has been implicated in inhibition of yolk proteases, we performed in vitro enzymatic assays with a cysteine protease to test whether it would be inhibited by PolyP. This cysteine protease is prominent in Anticarsia egg homogenates. Accordingly, short chain PolyP was a potent inhibitor of cysteine protease. We thereby suggest that PolyP hydrolysis by agAP is a triggering mechanism of yolk mobilization in A. gemmatalis.

Microscopic and molecular characterization of ovarian follicle atresia in Rhodnius prolixus Stahl under immune challenge

In this work we characterized the degenerative process of ovarian follicles of the bug Rhodnius prolixus challenged with the non-entomopathogenic fungus Aspergillus niger. An injection of A. niger conidia directly into the hemocoel of adult R. prolixus females at the onset of vitellogenesis caused no effect on host lifespan but elicited a net reduction in egg batch size. Direct inspection of ovaries from the mycosed insects revealed that fungal challenge led to atresia of the vitellogenic follicles. Light microscopy and DAPI staining showed follicle shrinkage, ooplasm alteration and disorganization of the monolayer of follicle cells in the atretic follicles. Transmission electron microscopy of thin sections of follicle epithelium also showed nuclei with condensed chromatin, electron dense mitochondria and large autophagic vacuoles. Occurrence of apoptosis of follicle cells in these follicles was visualized by TUNEL labeling. Resorption of the yolk involved an increase in protease activities (aspartyl and cysteinyl proteases) which were associated with precocious acidification of yolk granules and degradation of yolk protein content. The role of follicle atresia in nonspecific host-pathogen associations and the origin of protease activity that led to yolk resorption are discussed.

Inorganic polyphosphate inhibits an aspartic protease-like activity in the eggs of Rhodnius prolixus (Stahl) and impairs yolk mobilization in vitro

Inorganic polyphosphate (poly P) is a polymer of phosphate residues that has been shown to act as modulator of some vertebrate cathepsins. In the egg yolk granules of Rhodnius prolixus, a cathepsin D is the main protease involved in yolk mobilization and is dependent on an activation by acid phosphatases. In this study, we showed a possible role of poly P stored inside yolk granules on the inhibition of cathepsin D and arrest of yolk mobilization during early embryogenesis of these insects. Enzymatic assays detected poly P stores inside the eggs of R. prolixus. We observed that micromolar poly P concentrations inhibited cathepsin D proteolytic activity using both synthetic peptides and homogenates of egg yolk as substrates. Poly P was a substrate for Rhodnius acid phosphatase and also a strong competitive inhibitor of a pNPPase activity. Fusion events have been suggested as important steps towards acid phosphatase transport to yolk granules. We observed that poly P levels in those compartments were reduced after in vitro fusion assays and that the remaining poly P did not have the same cathepsin D inhibition activity after fusion. Our results are consistent with the hypothesis that poly P is a cathepsin D inhibitor and a substrate for acid phosphatase inside yolk granules. It is possible that, once activated, acid phosphatase might degrade poly P, allowing cathepsin D to initiate yolk proteolysis. We, therefore, suggest that degradation of poly P might represent a new step toward yolk mobilization during embryogenesis of R. prolixus.

Interplay between acid phosphatase and cysteine proteases in mediating vitellin degradation during early embryogenesis of Periplaneta americana

In this work, we characterized the activities of two classes of proteases and AcP during early embryogenesis of Periplaneta americana. AcP activity was first detected at day 6 and reached a maximum level at day 10 of development. Using phosphoamino acids, phosphatase activity was shown to be directed only against phosphotyrosine at day 6 while at day 10 it was also active against phosphoserine. In parallel, two classes of proteases were detected and located within yolk granules: a clan CA-cysteine protease, which was inhibited by E-64, insensitive to CA 074 and activated by acidic pH at day 3; and a neutral serine protease, which was inhibited by aprotinin at day 6. Assays of vitellin (Vt) degradation evidenced that incubations at neutral pH induced slight proteolysis, while the incubations at acidic pH did not result in Vt degradation. However, pre-incubations of Vt with AcP increased the levels of Vt acidic proteolysis and this could be inhibited by the addition of phosphatase inhibitors. On the other hand, the same pre-incubations showed no effects on the profile of degradation at neutral pH. We propose that AcP and cysteine protease cooperate to assure Vt breakdown during early embryogenesis of P. americana.

Calcium-regulated fusion of yolk granules is important for yolk degradation during early embryogenesis of Rhodnius prolixusStahl

This study examined the process of membrane fusion of yolk granules (YGs)during early embryogenesis of Rhodnius prolixus. We show that eggs collected at days 0 and 3 after oviposition contain different populations of YGs, for example day-3 eggs are enriched in large YGs (LYGs). Day-3 eggs also contain the highest free [Ca2+] during early embryogenesis of this insect. In vitro incubations of day-0 YGs with [Ca2+]similar to those found in day-3 eggs resulted in the formation of LYGs, as observed in vivo. Fractionation of LYGs and small YGs (SYGs) and their subsequent incubation with the fluorescent membrane marker PKH67 showed a calcium-dependent transference of fluorescence from SYGs to LYGs, possibly as the result of membrane fusion. Acid phosphatase and H+-PPase activities were remarkably increased in day-3 LYGs and in calcium-treated day-0 LYGs. Both fractions were found to contain vitellins as major components, and incubation of YGs with calcium induced yolk proteolysis in vitro. Altogether, our results suggest that calcium-induced membrane fusion events take part in yolk degradation, leading to the assembly of the yolk mobilization machinery.

Characterization of a tyrosine phosphatase activity in the oogenesis of Periplaneta americana

In this work, phosphatase activity was characterized in the ovary and the haemolymph of Periplaneta americana. The optimum pH for these activities was 4.0, and a temperature of 44 degrees C was ideal for the maximal enzyme activity. The phosphatase activities were inhibited by NaF, sodium tartrate, Pi, sodium orthovanadate, and ammonium molybdate. The ovarian phosphatase activity at pH 4.0 was almost exclusive against phosphotyrosine, with little or no effect on the residues of phosphoserine or phosphothreonine. These results indicate that this phosphatase activity is due to the presence of an acid tyrosine phosphatase. The phosphatase activities of acid extracts from P. americana ovaries (OEX) and an acid extract from P. americana haemolymph (HEX) were analyzed in non-denaturant gel electrophoresis using an analog substrate beta-naphtyl phosphate. The gel revealed two bands with phosphatase activity in the ovary and one band in the haemolymph; these bands were excised and submitted to a 10% SDS-PAGE showing a single 70-kDa polypeptide in both samples. Histochemistry of the ovary with alpha-naphtyl phosphate for localization of acid phosphatase activity showed mainly labeling associated to the oocyte peripheral vesicles, basal lamina, and between follicle cells. Electron microscopy analysis showed that acid phosphatase was localized in small peripheral vesicles in the oocyte, but not inside yolk granules. The possible role of this phosphatase during oogenesis and embryogenesis is also discussed in this article.

Heparan sulfate is the main sulfated glycosaminoglycan species in internal organs of the male cockroach, Periplaneta americana

Sulfated glycosaminoglycans (GAGs) were isolated and characterized in thoracic muscle, fat body, whole digestive tract (stomach+intestine) and reproductive tract of adult male cockroaches, Periplaneta americana. Heparan sulfate (HS) was the predominant sulfated GAG species in the tissues analyzed, corresponding to more than 90% of the total sulfated GAG content. In both the thoracic muscle and fat body it was the only sulfated GAG species detected. We also determined the location of sulfated GAGs in most of these organs by histochemical analysis using 1,9-dimethylmethylene blue. In the thoracic muscle, sulfated GAG metachromatic staining was detected only in the connective tissue that surrounds the muscle bundles or fascicles. In the intestinal tract, metachromatic staining was observed in both epithelial and lining columnar cells. Only spermatozoa presented metachromatic material in the male reproductive tract. Since, HS corresponds to 90-100% of total sulfated GAGs in these tissues, the metachromatic staining specifically reflects the location of this particular sulfated GAG in these organs. In conclusion, the present study extends previous observations on the GAG composition in cockroaches providing new information on the tissue distribution and location of HS in several internal organs of adult males of the cockroach P. americana.

A new model for proton pumping in animal cells: the role of pyrophosphate

The H+-PPase activity was characterized in membrane fractions of ovary and eggs of Rhodnius prolixus. This activity is totally dependent on Mg2+, independent of K+ and strongly inhibited by NaF, IDP and Ca2+. The membrane proteins of eggs were analyzed by western blot using antibodies to the H+-PPase from Arabidopsis thaliana. The immunostain was associated with a single 65-kDa polypeptide. This polypeptide was immunolocalized in yolk granule membranes by optical and transmission electron microscopy. We describe the acidification of yolk granules in the presence of PPi and ATP. This acidification is inhibited in the presence of NAF, Ca2+ and antibodies against H+-PPase. These data show for the first time in animal cells that acidification of yolk granules involves an H+-PPase as well as H+-ATPase.

The role of eicosanoids on Rhodnius heme-binding protein (RHBP) endocytosis by Rhodnius prolixus ovaries

The participation of eicosanoids and second messengers on the regulation of RHBP endocytosis by the ovaries was investigated, using [(125)I]RHBP in experiments in vivo and in vitro. Addition of PGE(2) (one of the products of the cyclooxygenase pathway) decreased in vitro the uptake of RHBP by 35%. The rate of RHBP endocytosis increased in the presence of indomethacin, a potent cyclooxigenase inhibitor, up to 50% in vitro and up to 55% in vivo, thus giving support to the role of cyclooxygenase derivatives on endocytosis regulation. The amount of PGE(2) secreted to the culture medium by the cells of Rhodnius prolixus ovaries was 1.1 ng/ovary following RHBP uptake assay. The amount of PGE(2) decreases approximately 25% in the presence of 5 microM indomethacin. Using a scanning electron microscope we have observed that neither the surface area nor the patencies of follicle cells were affected by treatment with indomethacin, thus suggesting that, its effect is elicited in the oocyte. Finally, we have identified two ovarian peptides that were dephosphorylated after the indomethacin treatment (18 and 25 kDa). Taken together these data show that local mediators such as eicosanoids act upon the oocytes controlling RHBP endocytosis, perhaps using the protein phosphorylation signal transduction pathway.