In vitro elicitation, isolation, and characterization of conessine biomolecule from Holarrhena antidysenterica (L.) Wall. callus and its larvicidal activity against malaria vector, Anopheles stephensi Liston

Modulation in the enzymatic antioxidants, MDA level and elicitation in conessine biomolecule in Holarrhena pubescens (medicinal tree) cultures exposed to different heavy metals: Ni, Co, Cr and As

Nodal explants of Holarrhena pubescens, an important medicinal tree, were cultured on Murashige and Skoog's medium (MS) containing 15 µM BA (control) alone and on medium supplemented with different concentrations (0, 1, 5, 25, 50, 100 and 200 mg/L) of heavy metals such as NiCl2, CoCl2, As2O3 and CrO3 to study their toxic effect. After 28 days of treatments, the nodal segments were harvested to assess the average number of shoots per explants, average shoot length, malondialdehyde content, proline content, conessine accumulation and antioxidant enzymatic activity. Among all the metals tried, best morphogenic response was achieved at 5 mg/L CrO3 where 80% culture differentiated an average of 3.21 ± 0.08 shoots per explant having 0.95 ± 0.018 cm average shoot length. Highest concentration (200 mg/L) of all the heavy metals proved lethal for morphogenesis. Maximum inhibition in average shoot number and average shoot length was observed in nodal explants treated with 25 mg/L As2O3 where an average of 0.49 ± 0.047 shoots having an average shoot length of 0.3 ± 0.02 cm. Contrarily, addition of heavy metals in culture medium proved strong elicitors, exhibiting significant enhancement in the biosynthesis of conessine, an important bioactive compound. HPLC analysis of the crude extract of in vitro grown untreated nodal cultures revealed an average of 117.06 ± 2.59 µg/g d. w. of conessine, whereas those treated with 100 mg/L of CoCl2 accounted for 297.1 ± 7.76 µg/g d. w. (an increase of 156% over control). Among the heavy metals tried, CoCl2 proved to be the best for conessine enhancement which was in the order of CoCl2 > Cr2O3 > NiCl2 > As2O3 in the nodal explants. Concomitantly, MDA content, the antioxidant enzymes activities of catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GR) and ascorbate peroxidase (APX) were also observed to be differentially expressed with the increase in the heavy metals concentration from 1 to 200 mg/L. Free proline, too, increased up to 3.5-fold over control. The results obtained during the present investigation revealed that the overall response of the nodal explants in terms of morphogenesis, conessine content and antioxidant enzyme activities was metal specific as well as dose dependent.

Holanamine, a Steroidal Alkaloid from the Bark of Holarrhena pubescens Wall. ex G. Don Inhibits the Growth of Leishmania donovani by Targeting DNA Topoisomerase 1B

Leishmaniasis is a group of neglected tropical diseases (NTDs) caused by about 20 species of obligate intracellular protozoan parasites of the genus Leishmania, which occurs in cutaneous, mucocutaneous, and visceral forms. Many researchers have sought to utilize natural products for novel and effective treatments to combat many infectious diseases, including leishmaniasis. Holarrhena pubescens Wall. ex G. Don (Apocynaceae) bark is a rich source of bioactive steroidal alkaloids. The total alkaloidal extract (IC₅₀ 6.12 ± 0.117 μg/mL), and the isolated alkaloid, holanamine, showed significant antileishmanial activity (IC₅₀ 2.66 ± 0.112 μM against AG83 and 3.80 ± 0.126 μM against BHU-575) against the Leishmania donovani parasite, better than miltefosine (IC₅₀ 19.61 ± 0.093 μM against AG83 and 23.20 ± 0.094 μM against BHU-575). Holanamine inhibited the L. donovani topoisomerase 1B (LdToP1B) in a non-competitive manner (IC₅₀ 2.81 ± 0.105 μM), indicating that it interacts with the free enzyme and enzyme-DNA complex without inhibiting human topoisomerase. Hydrogen bonding and hydrophobic interactions of holanamine with the N-terminal and hinge region of the large subunit of LTop1B is responsible for its potent antileishmanial activity, as shown by docking studies. Treatment with holanamine causes apoptotic-like cell death by generating cellular and mitochondrial reactive oxygen species, disrupting the mitochondrial membrane potential and inducing ultrastructural alterations in the promastigotes. Holanamine effectively clears intracellular amastigotes but minimally affects host macrophages with no significant cytotoxicity in HEK 293 and L929 cell lines. Thus, our studies show that holanamine can further be used to develop effective antileishmanial agents against evolving drug-resistant parasites.

Naturally Occurring Compounds With Larvicidal Activity Against Malaria Mosquitoes

Female Anopheles mosquitoes transmit Plasmodium parasites that cause human malaria. Currently, vector control is the most widely deployed approach to reduce mosquito population and hence disease transmission. This relies on use of insecticide-based interventions including Long-lasting Insecticide-treated Nets (LLINs) and Indoor Residual Spraying (IRS) where scale-up has contributed to a dramatic decline in malaria deaths and morbidity over the past decade. Challenges to their effective use include the emergence and spread of insecticide resistance by malaria vector populations coupled with the inability to curb outdoor transmission. Under these situations, use of larvicides through larval source management (LSM) can complement these existing measures. The need to minimize environmental impact and effect on non-target organisms has spurred interest in the development of eco-friendly larvicides of natural origin. Here, we review literature published in the last five years to highlight compounds of natural origin found to exhibit larvicidal activity against malaria mosquitoes. Specifically, the larvicidal activity of different classes of compounds is discussed including their effect on non-target organisms. Additionally, we provide suggestions for future research into mosquito larvicides including the use of chemical synthesis to improve the bioactivity of known natural compounds.