Structural Diversity and Biological Activities of Cyclic Depsipeptides from Fungi

Cyclic depsipeptides (CDPs) are cyclopeptides in which amide groups are replaced by corresponding lactone bonds due to the presence of a hydroxylated carboxylic acid in the peptide structure. These peptides sometimes display additional chemical modifications, including unusual amino acid residues in their structures. This review highlights the occurrence, structures and biological activities of the fungal CDPs reported until October 2017. About 352 fungal CDPs belonging to the groups of cyclic tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-, and tridecadepsipeptides have been isolated from fungi. These metabolites are mainly reported from the genera Acremonium, Alternaria, Aspergillus, Beauveria, Fusarium, Isaria, Metarhizium, Penicillium, and Rosellina. They are known to exhibit various biological activities such as cytotoxic, phytotoxic, antimicrobial, antiviral, anthelmintic, insecticidal, antimalarial, antitumoral and enzyme-inhibitory activities. Some CDPs (i.e., PF1022A, enniatins and destruxins) have been applied as pharmaceuticals and agrochemicals.


Introduction
Cyclic depsipeptides (CDPs), also known as cyclodepsipeptides or peptolides, are cyclooligomers in which one or more amino acid is replaced by a hydroxylated carboxylic acid, resulting in the formation of at least one lactone bond in the core ring. They are biosynthesized by non-ribosomal peptide synthetases (NRPS) in combination with either polyketide synthase (PKS) or fatty acid (FA) synthase enzyme systems [1][2][3]. CDPs are widely distributed in bacteria [4], fungi [1], plants [5,6], algae [7], sponges [8], and other marine organisms [9][10][11][12][13]. Here, we focus on fungal CDPs which include cyclic tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-, and tridecadepsipeptides though fungi can produce large amounts of cyclic peptides without any lactone bond in the core ring [14,15]. Some fungal CDPs such as beauvericins, destruxins, enniatins have been well characterized [16][17][18][19]. Special reviews covering chemical synthesis [16], biosynthesis [20], chemical classification [3], as well as applications [21,22] of fungal CDPs are also available. In this review, we describe the occurrence, biological activities, and structures of all hitherto reported fungal CDPs to assess which of them merit further study for purposes of drug development as well as for clarification of their physiological and ecological functions. We still classify fungal CDPs based on the total amounts of amino and hydroxylated carboxylic acids though a review about the classification of CDPs based on the hydroxylated carboxylic acid(s) involved in the ring lacone has just been published [3].

Cyclic Tridepsipeptides
Cyclic tridepsipeptides usually contain two amino acids and one hydroxylated carboxylic acid. They were found in the genera Acremonium, Calcarisporium, Fusarium, Phomopsis and Ramalina.
The occurrence and biological activities of fungal cyclic tridepsipeptides are listed in Table 1, and their structures are shown in Figure 1.
PM181110 (9) was identified from the endophytic fungus Phomopsis glabrae isolated from the leaves of Pongamia pinnata, and exhibited anticancer activity against 40 human cancer cell lines with a mean IC 50 value of 0.089 µM. The structure of this compound has a disulfide ring, which possibly contributed to the biological activity [26].
Stereocalpin A (10) was isolated from the endophytic fungus Ramalina terebrata associated with the Antarctic lichen Stereocaulon alpinum. This CDP is unique in that its structure contains a 5-hydroxy-2,4-dimethyl-3-oxo-octanoic acid. It showed moderate cytotoxic activity against three human solid tumor cell lines (i.e., colon carcinoma cell line HT-29, skin carcinoma cell line B16/F10, and liver carcinoma cell line HepG2), and weak inhibitory activity against protein tyrosine phosphatase 1B (PTP1B) [27]. Further investigation of the mechanism showed that stereocalpin A (10) inhibited the expression of adhesion molecules in activated muscle cells. These results suggest that this compound has the potential to exert a protective effect by modulating inflammation within the atherosclerotic lesion [28]. Table 1. Fungal cyclic tridepsipeptides and their biological activities.

Cyclic Tetradepsipeptides
Forty nine cyclic tetradepsipeptides have been isolated from fungi so far. They have been found mainly in the genera Alternaria, Aspergillus, Beauveria, Fusarium, Hypoxylon, and Penicillium. Their occurrences in fungi, and biological activities are listed in Table 2, and the structures are provided in Figure 2.
15G256γ (11), δ (12) and ε (13) were isolated from the marine fungus Hypoxylon oceanicum (LL-15G256) [29,30]. They showed moderate antifungal activity against the plant pathogenic fungi in greenhouse tests and human fungal pathogens in vitro. Microscopic examination of treated fungi suggested that the compounds displayed inhibition on cell wall biosynthesis [31].
Clavatustides A (49) and B (50) were identified from the cultured mycelia and broth of Aspergillus clavatus C2WU. The fungus was isolated from the crab Xenograpsus testudinatus, which lived at extreme, toxic habitat around the sulphur-rich hydrothermal vents in Taiwan Kueishantao. Both compounds suppressed the proliferation of hepatocellular carcinoma (HCC) cell lines (HepG2, SMMC-7721 and Bel-7402), and induced an accumulation of HepG2 cells in G1 phase and reduction of cells in S phase [40]. CCNE2 (cyclin E2) was proved to be the key regulator of clavatustide B-induced G1-S transition blocking in several cancer cell lines by using real-time PCR [41].
Fusaristatins A (51) and B (52) were identified in the endophytic fungus Fusarium sp. YG-45. Both compounds showed a moderate inhibitory effect on topoisomerases I and II. They also showed the growth-inhibitory activity toward lung cancer cells LU 65 [42]. Fusaristatin A (51) also displayed an inhibitory effect on the fungus Glomerella acutata [43].
Destruxins are mainly isolated from the entomopathogenic fungus Metarhizium anisopliae. More than 35 destruxin analogs have been identified in this fungus [19]. Destruxin A (141) can induce and bind heat shock proteins (HSPs) in Bombyx mori Bm12 cells [90]. Most of destruxins exhibit insecticidal and phytotoxic activities. Other biological activities include antimicrobial, antitrypanosomal, cytotoxic, immunosuppressant, antiproliferative and antiviral acitivites. Destruxins act as V-ATPase inhibitors and provide a basis for the development of new drugs to against osteoporosis, cancer, or as the biological control agents [16,19]. Destrusins cause an initial tetanic paralysis, which is attributed to muscle depolarization by direct opening of Ca 2+ channels in the membrane [16]. They can act as V-ATPase inhibitors, and modulate the antiapoptotic funcition of Bcl-xL through their inherent ability to inhibit the V-ATPase activity as a result of a caspase-independent pathway [19].
Enniatins have been isolated largely from Fusarium species, although they were isolated from other fungal genera, such as Verticillium and Halosarpheia [18]. About 30 enniatins have been isolated and characterized, either as a single compound or mixtures of inseparable homologs. Structurally, these depsipeptides are biosynthesized by a multifunctional enzyme, termed enniatin synthetase, and composed of six residues that alternate between N-methyl amino acids and hydroxylated carboxylic acids [18].
Isoisariin B (240) was isolated from the entomopathogenic fungus Beauveria felina. This compound was active against the pest-insect Sitophilus spp. with an LD 50 value of 10 µg/mL [96]. Other isariin analogs including isariins A (231) Nodupeptide (242) was isolated from the gut of the insect Riptortus pedestris. This compound displayed insecticidal activity against rice brown planthopper (Nilaparvata lugens) with an LD 50 value of 70 ng/larva, and inhibitory activity towards the drug-resistant human pathogenic bacterium Pseudomonas aeruginosa with the MIC value (5.0 µM) comparable to that (3.2 µM) of the positive control ciprofloxacin [100].
Paecilodepsipeptide A (also namely gliotide, 248) was first obtained from the marine-derived fungus Gliocladium sp. from the alga Durvillaea antarctica [101], and later isolated from the insect pathogenic fungus Paecilomyces cinnamomeus BCC 9616 [102]. This compound exhibited antimalarial activity on Plasmodium falciparum K1 and cytotoxic activity on KB and BC cell lines [102].
Pseudodestruxins A (249) and B (250) were obtained from the coprophilous fungus Nigrosabulum globosum isolated from sheep dung. Both had antibacterial activity on Bacillus subtilis and Staphylococcus aureus [103].
Roseotoxin B (259) from Trichothecium roseum improved allergic contact dermatitis through a unique anti-inflammatory mechanism involving excessive activation of autophagy in activated T lymphocytes [104].

Cyclic Heptadepsipeptides
The occurrences and biological activities of fungal cyclic heptadepsipeptides are shown in Table 5, and their structures are provided in Figure 5.
Cordycommunin (277) was obtained from the insect pathogenic fungus Ophiocordyceps communis BCC16475. This compound exhibited inhibitory activity on Mycobacterium tuberculosis H37Ra. It also showed weak cytotoxic activity on KB cells [180].
Fusaripeptide A (278) was obtained from the endophytic fungus Fasarium sp. from the roots of Mentha longifolia L. growing in Saudi Arabia. It exhibited antifungal, anti-malarial and cytotoxic activities [181].
W493 A (285), B (286), C (287) and D (288) were obtained from the endophytic fungus Fusarium sp. isolated from the mangrove plant Ceriops tagal. Both W493 A (285) and B (286) exhibited moderate activity against the fungus Cladosporium cladosporiodes and weak antitumor activity against the human ovarian cancer cell line A2780 [184]. W493 A and B were also isolated from Fusarium sp. and showed strong antifungal activity against Venturia inaequalis, Monilinia mali, and Cochliobolus miyabeanus [185].

Cyclic Heptadepsipeptides
The occurrences and biological activities of fungal cyclic heptadepsipeptides are shown in Table 5, and their structures are provided in Figure 5.
Cordycommunin (277) was obtained from the insect pathogenic fungus Ophiocordyceps communis BCC16475. This compound exhibited inhibitory activity on Mycobacterium tuberculosis H37Ra. It also showed weak cytotoxic activity on KB cells [180].
Fusaripeptide A (278) was obtained from the endophytic fungus Fasarium sp. from the roots of Mentha longifolia L. growing in Saudi Arabia. It exhibited antifungal, anti-malarial and cytotoxic activities [181].
W493 A (285), B (286), C (287) and D (288) were obtained from the endophytic fungus Fusarium sp. isolated from the mangrove plant Ceriops tagal. Both W493 A (285) and B (286) exhibited moderate activity against the fungus Cladosporium cladosporiodes and weak antitumor activity against the human ovarian cancer cell line A2780 [184]. W493 A and B were also isolated from Fusarium sp. and showed strong antifungal activity against Venturia inaequalis, Monilinia mali, and Cochliobolus miyabeanus [185].

Cyclic Octadepsipeptides
The occurrences and biological activites of reported fungal cyclic octadepsipeptides are listed in Table 6, and their structures are shown in Figure 6.
Phaeofungin (301), which was isolated from the endophytic fungus Phaeosphaeria sp. from living stems and leaves of Sedum sp. (Crassulaceae), was discovered by application of reverse genetics technology, using the Candida albicans fitness test (CaFT). This compound caused ATP release in wild-type Candida albicans strains. It showed modest antifungal activity with the MICs for Candida albicans, Aspergillus fumigatus, and Trichophyton mentagrophytes as 16, 8 and 4 μg/mL, respectively [196].

Cyclic Octadepsipeptides
The occurrences and biological activites of reported fungal cyclic octadepsipeptides are listed in Table 6, and their structures are shown in Figure 6.
Phaeofungin (301), which was isolated from the endophytic fungus Phaeosphaeria sp. from living stems and leaves of Sedum sp. (Crassulaceae), was discovered by application of reverse genetics technology, using the Candida albicans fitness test (CaFT). This compound caused ATP release in wild-type Candida albicans strains. It showed modest antifungal activity with the MICs for Candida albicans, Aspergillus fumigatus, and Trichophyton mentagrophytes as 16, 8 and 4 µg/mL, respectively [196].

Cyclic Nonadepsipeptides
The origins and biological activities of fungal cyclic nonadepsipeptides are listed in Table 7, and their structures are provided in Figure 7. Aureobasins were isolated from the black yeast Aureobasidium pullulans R106 from the leaf collected at Tsushima of Japan. They are composed of one hydroxylated carboxylic acid and eight amino acids, and 29 aureobasidin analogs (305-333) have been isolated from this fungus [203][204][205][206]. They showed good in vitro activity against all Candida species and Cryptococcus neoformans, in vivo activity against murine systemic candidiasis, and had low toxicity. They also showed inhibitory activity on inositol phosphorylceramide synthase [207].

Name Fungus and Its Origin Biological Activity References
Aureobasidin A (305) Aureobasidium pullulans from a leaf collected at Tsushima of Japan Antifungal activity; Inhibitory activity on Candida planktonic and biofilm cells [203,211,212] Aureobasidin B (306) Aureobasidium pullulans from a leaf collected at Tsushima of Japan Antifungal activity [204,213] Aureobasidin C (307) Aureobasidium pullulans from a leaf collected at Tsushima of Japan Antifungal activity [204,213] Aureobasidin D (308) Aureobasidium pullulans from a leaf collected at Tsushima of Japan Antifungal activity [204,213]

Cyclic Nonadepsipeptides
The origins and biological activities of fungal cyclic nonadepsipeptides are listed in Table 7, and their structures are provided in Figure 7. Aureobasins were isolated from the black yeast Aureobasidium pullulans R106 from the leaf collected at Tsushima of Japan. They are composed of one hydroxylated carboxylic acid and eight amino acids, and 29 aureobasidin analogs (305-333) have been isolated from this fungus [203][204][205][206]. They showed good in vitro activity against all Candida species and Cryptococcus neoformans, in vivo activity against murine systemic candidiasis, and had low toxicity. They also showed inhibitory activity on inositol phosphorylceramide synthase [207].

Name Fungus and Its Origin Biological Activity References
Aureobasidin A (305) Aureobasidium pullulans from a leaf collected at Tsushima of Japan Antifungal activity; Inhibitory activity on Candida planktonic and biofilm cells [203,211,212] Aureobasidin B (306) Aureobasidium pullulans from a leaf collected at Tsushima of Japan Antifungal activity [204,213] Aureobasidin C (307) Aureobasidium pullulans from a leaf collected at Tsushima of Japan Antifungal activity [204,213] Aureobasidin D (308) Aureobasidium pullulans from a leaf collected at Tsushima of Japan Antifungal activity [204,213]

Conclusions and Future Perspectives
In this review, we describe the chemistry and biology of the CDPs discovered from fungi during the past 50 years. It is worth mentioning that more and more CDPs have been isolated from plant endophytic and marine-derived fungi which indicate that plant-derived endophytic and marinedrived fungi are the mines of biologically active natural products [10,13,[226][227][228]. Some invertebrate derived CDPs (e.g., from sponge origin) are actually synthesized by the symbiotic microorganisms [229]. In addition, some minor or new CDPs have been identified in fungi with the application of new techniques such as LC-MS/MS [230], reverse genetics [196], genomics [138], epigenetic manipulation [62], and combinatorial biosynthesis [231,232].

Conclusions and Future Perspectives
In this review, we describe the chemistry and biology of the CDPs discovered from fungi during the past 50 years. It is worth mentioning that more and more CDPs have been isolated from plant endophytic and marine-derived fungi which indicate that plant-derived endophytic and marine-drived fungi are the mines of biologically active natural products [10,13,[226][227][228]. Some invertebrate derived CDPs (e.g., from sponge origin) are actually synthesized by the symbiotic microorganisms [229]. In addition, some minor or new CDPs have been identified in fungi with the application of new techniques such as LC-MS/MS [230], reverse genetics [196], genomics [138], epigenetic manipulation [62], and combinatorial biosynthesis [231,232].
Some fungal CDPs are currently in clinical use or have entered human clinical trials as antibiotic or anticancer agents. Some have been developed into commercial products [18,19,22]. The noteworthy example is the anthelmintic agent emodepside which is a semisynthetic derivative of PF1022A (293), a cyclic octadepsipeptide from the endophytic fungus Rosellina sp. PF1022 derived from the leaves of Camellia japonica [191]. Emodepside binds to a presynaptic latrophilin receptor and interacts with a calcium-activated potassium channel. Both modes of action cause paralysis and death of the nematode [242]. It is employed against gastrointestinal and extraintestinal parasites such as nematodes in veterinary medicine [193]. Another example is fusafungine, a mixture of enniatins, which is an antibacterial for the treatment of rhinosinusitis in nasal spray [18]. However, fusafungine has been recently withdrawn from the EU market since enniatins have been previously identified as mycotoxins which pose a potential health hazard on humans or animals [243][244][245]. The third example is the direct application of destruxins as insecticidal agents [19]. Destruxins were isolated from a variety of fungi such as Metarrhizium anisopliae [16], Beauveria felina [123], and Ophiocordyceps coccidiicola [128]. With the increasing understanding of the biosynthetic pathways of some fungal CDPs, we can rationally design bioengineering approaches such as chemoenzymatics, mutasynthesis, site-directed mutagenesis, and combinatorial biosynthesis. We may be able to effectively not only increase the yields of bioactive CDPs, but also block the biosynthesis of some toxic depsipeptides [231,246].