Most of the drugs currently available for the treatment of trypanosomiases have an ancient origin and high toxicity. Others, despite being specific and efficient for the early and advanced stages of infection, depend on an enzyme or membrane transporter of the parasite for their activation. The latter generally implies a probability of resistance development over time. For all these reasons, there is a need to develop or search compounds that will overcome these limitations observed in anti-trypanosome drugs available on the market. Although this search may seem very demanding, these “ideal drug candidates” could be found in various natural sources. This will be discussed in more depth in the next section.
4. AMPs with Antiparasitic Activity
Several studies have shown the antiparasitic effect of some AMPs
[43][44][45][35,107,108], including activity against parasites that cause important tropical diseases
[46][109] (
Figure 1). Many of these AMPs have been isolated from various vertebrate and invertebrate hosts of these parasites
[44][47][48][107,110,111].
Plasmodium is the parasite on which most studies with AMPs have been carried out
[49][112]. In this protist, many natural AMPs act primarily by disrupting the integrity of cell membranes
[50][51][52][53][54][55][117,118,119,120,121,122]. However, some others can interfere with other important cellular processes of the parasite. In
Plasmodium berghei, some fungal AMPs have an inhibitory effect on histone deacetylase (HDA), thus inducing histone hypermethylation and subsequent alteration of gene expression in the parasite
[56][123]. Other AMPs derived from Gram-positive bacteria, such as epoxomicin and derivatives of the natural cyclic oligopeptide thiostrepton, have an inhibitory effect on protein synthesis and turnover, due to their binding to and inhibition of catalytic activity of proteasome β subunits (20S)
[57][58][124,125]. Additionally, thiostrepton can inhibit mRNA translation in the apicoplast through its binding to the plasmodial organellar rRNA promoting structural alterations that prevent its function during protein synthesis
[58][59][125,126]. Importantly, antimalarial activities have been attributed to some AMPs with semi-synthetic and synthetic origin. Synthetic AMPs inhibit the plasmodial cysteine protease falcipain and aspartic proteases plasmepsin I and plasmepsin II, involved in hemoglobin hydrolysis and hemozoin formation, thus interfering with parasite metabolism and growth
[60][61][62][127,128,129]. Notably, some synthetic peptides have also shown an effect on some enzymes such as topoisomerase I, affecting the parasite’s DNA metabolism
[63][130]. Several of these AMPs not only have antiplasmodial activity against different developmental stages of some
Plasmodium species (
P. falciparum,
P. berghei, and
P. yoelii nigeriensis) in vitro conditions
[54][57][64][121,124,131], but are also effective at high parasitemia in an animal model
[55][122].
In helminths, studies have focused primarily on
Schistosoma and
Brugia. In these parasites, AMPs have effects on motility, development, egg deposition, and the integumentary surface
[65][66][67][68][69][136,137,138,139,140]. In
Brugia pahangi, synthetic cecropins A/B, AMPs from insect hemolymph, attenuate microfilariae mobility and larval development in adult female
Aedes aegypti [65][136]. In
Schistosoma, dermaseptin, a peptide isolated from frogs, can synergistically interact with other natural compounds and contribute to parasite killing and infection control. In combination with piplartine, an amide alkaloid of
Piper longum L. (long piper), dermaseptin not only exerts activity against the
Schistosoma mansoni (
S. mansoni) stages (schistosomula and adult) and affects the reproductive fitness of adult worms, but also induces structural alterations of the tegument and extensive destruction of the tubercles
[66][67][137,138]. Although the anthelmintic mechanism of AMPs has not been elucidated, it has been proposed that disruption of cell structure by pore formation by direct interaction with the lipid bilayer seems to be the most likely
[65][67][70][71][136,138,141,142]. It should be noted that the integument is essential for the survival of the helminth parasites, since it is involved in nutrient absorption and in the interaction with the host
[72][73][74][75][143,144,145,146]. In both
Brugia and
Schistosoma, divalent metal transporter 1 (DMT1) molecules are present in the integument and are essential for the absorption of iron, an essential ion for the development and reproduction of these parasites
[73][75][76][144,146,147]. In this sense, directing AMPs against the tegument of these parasites could be a good anthelmintic strategy.
Unlike in apicomplexan parasites, the AMPs tested so far on helminths exert their antiparasitic action at micromolar concentrations
[65][66][67][136,137,138]. Some AMPs with antimicrobial properties have been discovered in helminths
[71][77][78][142,148,149]. In
S. mansoni, an AMP called schistocins has been obtained from the protein SmKI-1, a key protein for the survival of this nematode, which has activity against
Schistosoma itself
[71][142]. Likewise, putative neuropeptides derived from this parasite alter the behavior of the cercariae stage, therefore their use has been proposed as strategy for the control of the infection
[69][140].
In trypanosomes such as
Trypanosoma evansi and
Trypanosoma equiperdum, causing surra and dourine in animals, some AMPs have been shown to exert an trypanocidal effect; hence, they have been proposed for use in new treatment strategies of trypanosomiasis in animals
[79][80][150,151]. Furthermore, AMPs isolated from triatomine hemolymph have been shown to have trypanolytic activity against different strains of
Trypanosoma rangeli, an infectious but non-pathogenic human parasite
[81][152]. In these trypanosomes, AMPs exert their action through different mechanisms, including plasma membrane permeabilization, mitochondrial alteration, and parasite lysis
[79][80][81][150,151,152].
5. Antimicrobial Peptides against Kinetoplastids Causing Neglected Tropical Diseases
5.1. AMPs against T. brucei
Many of the AMPs that are active against
T. brucei are produced by a wide variety of organisms, including mammals and the insect vector
[82][83][16,153]. These can carry out their action extracellularly, by plasma membrane disturbance, or intracellularly, by altering the function of some intracellular compartments
[82][82][83][84][16,16,153,154] (
Figure 23).
Figure 23. Mechanisms of action of antimicrobial peptides (AMPs) against
T. brucei. (
A). AMPs derived from the vertebrate host and insect vector. Peptides isolated from both vectors and hosts exert their trypanocidal effect through membrane perturbation and induction of cell lysis. (
B). Mechanism of action of neuropeptides (NPs). The killing of trypanosomes by NPs requires the NPs to be endocytosed through the flagellar pocket and transported from the endosomes to the acidified lysosome, where they break the lysosomal bilayer membrane and accumulate in the cytoplasm. Once in the cytoplasm, their interference in various cellular processes contributes to morphological alterations and disturbance of organelles (glycosomes and mitochondrion), which ultimately lead to depletion of ATP and failure of the energy metabolism. (
C). AMPs isolated from natural sources (bacteria, fungi, and insects). (C.1,C.2). AMPs derived from fungi. The lipopeptide amphomycin, inhibits the biosynthesis of the glycolipid precursor of glycosylphosphatidylinositol (GPI) by which the variant surface glycoproteins (VSGs) are anchored to the plasma membrane of these parasites (C.1). Leucinostatins (A and B) and alamethicin act as ionophores and pore formers in the membranes, causing alteration of cellular homeostasis, ultimately leading to the death of the parasite. (C.3). AMPs isolated from bacteria. Bacteriocin AS-48 targets intracellular compartments without plasma membrane permeabilization. AS-48 may interact at the surface with VSGs and then promotes its internalization through a clathrin-mediated endocytic process. In the cytoplasm, it induces structural alterations and autophagy-like cell death. (C.4). AMPs derived from bee venom. Melittin induces an increased influx of Ca
2+ through the plasma membrane or increased release from acidocalcisomes. Excess Ca
2+ accumulated intracellularly is stored in the mitochondrion, causing a reduced mitochondrial membrane potential, disorganization of kinetoplast DNA, autophagy, and cell death. (
D). Synthetic AMPs. (D.1). SHPs intercalate and insert deeply into the plasma membrane, resulting in changes in the distribution of membrane components, increased membrane stiffness, loss of cell motility, and cell death. (D.2). For their part, the CPPs cross the membrane, accumulate in the cytoplasm and interfere with various cellular processes (such as inhibition of metabolic enzymes and RNA/DNA synthesis). Created with
BioRender.com (accessed on 23 November 2022).
5.2. AMPs against T. cruzi
The antiparasitic activity of AMPs has also been evaluated on
T. cruzi, using some peptides obtained from a variety of natural sources and others synthetically prepared
[15][85][86][87][88][89][90][17,22,103,106,178,184,185] (
Figure 34).
Figure 34. Mechanisms of action of antimicrobial peptides (AMPs) against
T. cruzi. (
A). AMPs derived from the insect vector. These AMPs carry out their activities by disturbing the plasma membrane and forming pores in it. (A.1). Trialysin induces cell lysis. (A.2). Def1.3 promotes morphological alterations, reduced viability, and inhibits growth of the parasites. (A.3). Cecropin A perforates the plasma membrane, causing cell lysis. (
B). AMPs derived from the human host. (B.1). Def-α-1 exerts its trypanocidal effect through membrane pore formation, cytoplasmic vacuolization, and the induction of nuclear and mitochondrial DNA fragmentation, and detachment and release of the flagellum, leading to parasite destruction. Preincubation of parasites with this peptide inhibits their infective ability and causes reduction of the parasitemia. (B.2). The neuropeptide VIP modulates the inflammatory response to
T. cruzi, reducing cardiac damage. (
C). AMPs derived from other natural sources (insects, reptiles, and amphibians). (C.1) Melittin induces structural changes (including disruption of the plasma membrane, structural changes in the mitochondrion, kinetoplast disorganization, and structural alterations of the flagellum), alteration of Ca
2+ homeostasis, and activation of different cell death pathways in the parasite. (C.2). Polybia-CP and MP carry out their trypanocidal effect through the promotion of ROS, mitochondrial dysfunction and apoptosis-like cell death. Additionally, MP can inhibit the glycolytic enzyme GAPDH. (C.3). BatxC and Ctn induce the formation of pores in the plasma membrane, promoting the formation of ROS, loss of the mitochondrial membrane potential, and cell death by necrosis. (C.4). Peptides dermaseptins 1/4 and phylloseptins 7/8 have trypanocidal activity through disruption of the plasma membrane and effect on several intracellular targets (such as protein and nucleic acids synthesis). (
D). AMPs derived from aquatic organisms. Peptides isolated from marine organisms (Tach and fragments from hemocyanin) have anti-
T. cruzi activity by causing structural alterations in the plasma membrane and the formation of pores, and subsequent activation of cell death by necrosis. (
E). Synthetic AMPs. (E.1). Tempz and Tempz-1 have toxicity against
T. cruzi through cytoplasmic alterations in the parasite. These alterations are related to chromatin condensation, mitochondrial cristae disorder, kinetoplast disorganization, and an increased number and degeneration of reservosomes. (E.2). For their part, DC1-3 lytic peptides carry out their trypanocidal activity by perforation of the plasma membrane and subsequent cell lysis. Some of these peptides decrease the infectivity of the parasite, as well as the parasitemia and mortality of mice infected with
T. cruzi. Created with
BioRender.com.
6. Conclusions
AMPs are small peptides that have been shown to possess activity against different strains of
T. cruzi and
T. brucei, exerting their specific effect through different mechanisms such as rupture of the plasma membrane, alteration of calcium homeostasis, inhibition of some metabolic pathways, disturbance of organelles, and activation of various cell death pathways. Many of them have been shown to carry out their activity against the different developmental stages of trypanosomes. Some of them may also have activity against other kinetoplastids such as
Leishmania spp. Additionally, most of them have no or only low toxicity towards mammalian cells and little anti-inflammatory effects. All these attributes render AMPs promising tools for the design of novel trypanocidal agents. It seems appropriate to consider them as candidates for further investigation and possible application as new therapeutic agents for trypanosomiasis and other diseases caused by kinetoplastids, either as an alternative or administered in complementary strategy to conventional treatments. Likewise, they could be used as a template for the design of analogous molecules with greater trypanocidal potency and/or reduced cytotoxicity on the host.