The protozoan parasite Leishmania spp. causes leishmaniases, a group of diseases creating serious health problems in many parts of the world with significant resistance to existing drugs. Insect derived antimicrobial peptides are promising alternatives to conventional drugs against several human disease-causing pathogens because they do not generate resistance. Halictine-2, a novel antimicrobial peptide from the venom of eusocial honeybee, Halictus sexcinctus showed significant anti-leishmanial activity in vitro, towards two life forms of the dimorphic parasite, the free-swimming infective metacyclic promastigotes and the intracellular amastigotes responsible for the systemic infection. The anti-leishmanial activity of the native peptide (P5S) was significantly enhanced by serine to threonine substitution at position 5 (P5T). The peptide showed a propensity to form α-helices after substitution at position-5, conferring amphipathicity. Distinct pores observed on the promastigote membrane after P5T exposure suggested a mechanism of disruption of cellular integrity. Biochemical alterations in the promastigotes after P5T exposure included generation of increased oxygen radicals with mitochondrial Ca2+ release, loss of mitochondrial membrane potential, reduction in total ATP content and increased mitochondrial mass, resulting in quick bioenergetic and chemiosmotic collapse leading to cell death characterized by DNA fragmentation. P5T was able to reduce intracellular amastigote burden in an in vitro model of Leishmania infection but did not alter the proinflammatory cytokines like TNF-α and IL-6. The ability of the P5T peptide to kill the Leishmania parasite with negligible haemolytic activity towards mouse macrophages and human erythrocytes respectively, demonstrates its potential to be considered as a future antileishmanial drug candidate.
Keywords: Antimicrobial peptide; Cell death; Combinatorial therapy; Halictine-2; Leishmania; Leishmanicidal activity.
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