Chemical Diversity and Biological Activities of Meroterpenoids from Marine Derived-Fungi: A Comprehensive Update

Meroterpenoids are a class of hybrid natural products, partially derived from a mixed terpenoid pathway. They possess remarkable structural features and relevant biological and pharmacological activities. Marine-derived fungi are a rich source of meroterpenoids featuring structural diversity varying from simple to complex molecular architectures. A combination of a structural variability and their myriad of bioactivities makes meroterpenoids an interesting class of naturally occurring compounds for chemical and pharmacological investigation. In this review, a comprehensive literature survey covering the period of 2009–2019, with 86 references, is presented focusing on chemistry and biological activities of various classes of meroterpenoids isolated from fungi obtained from different marine hosts and environments.


Introduction
Meroterpenoids are a large group of secondary metabolites of mixed biosynthetic origin, partially derived from mevalonate pathways. Another part of these metabolites can be derived from other biosynthetic pathways, most of which are polyketides and, to a lesser extent, nonpolyketides such as amino acids [1]. Meroterpenoids are widespread in nature, being isolated from terrestrial plants [2], marine invertebrates [3], and microorganisms such as fungi [4] and bacteria [5,6]. Fungi not only are the most prolific producers of meroterpenoids but also synthesize structurally diverse metabolites of this group with a wide range of biological and pharmacological activities [4]. Consistently, Geris and Simpson [4] published the first review of meroterpenoids produced by fungi in 2009, covering the period of 1968 to August 2008. This review provided information on isolation, structure elucidation and some biological activities, in addition to a detailed discussion of biosynthetic studies of 333 fungal meroterpenoids. However, in most cases, there was no indication if the fungi under study were from terrestrial or marine origin. In 2016, Matsuda and Abe published a comprehensive review of the biosynthesis of fungal meroterpenoids, updating the biosynthetic information previously discussed in the review by Geris and Simpson by summarizing the molecular basis, elucidated by modern techniques, of various classes of meroterpenoids [1]. On the other hand, it is interesting to note that despite the discovery of cephalosporins from the marine-derived fungus Cephalosporium acremonium (which is known today as Acremonium chrysogenum) in 1948 [7], the interest in the investigation of secondary metabolites from marine-derived fungi only started in the 90s, with only 15 marine fungal metabolites reported by 1992 [8]. However, with the renewed interest in fungal biodiversity of the marine environment, the number of the isolated compounds kept rising to 270 in 2002 [9], and ramped up to 690 during the period of 2006 to mid-2010 [10]. From the literature search, it is evident that meroterpenoids constitute an important class of structurally unique secondary metabolites with relevant biological and pharmacological activities produced by fungi from nearly every possible marine habitat including soil and sediments, marine invertebrates (e.g., sponges, corals, sea cucumbers), marine plants (e.g., algae, sea glass, mangroves), and marine vertebrates (fishes) [11]. Moreover, they also display a myriad of biological activities including antioxidant [12], cytotoxic [13][14][15], antimicrobial [16,17], antiviral [18,19], anti-inflammatory [20], and anti-Alzheimer [21]. Despite this extraordinary increase in the research on natural products from marine-derived fungi, there is no systematic review of meroterpenoids from marine-derived fungi to date. Therefore, this review focuses on the chemistry and relevant biological activities of 320 meroterpenoids from marine-derived fungi reported in the literature over the period of 2009 to December 2019. Contrary to the classification based on the types of polyketides adopted by Geris and Simpson [4], herein we grouped the reported meroterpenoids according to the terpenoid classes, i.e., hemiterpenes, monoterpenes, sesquiterpenes and diterpenes. In this review, the biosynthesis aspects of this class of compounds are not discussed as they have been extensively reviewed by Geris and Simpson [4] and then updated by Matsuda and Abe [1].

Chemistry and Biology of Meroterpenoids Isolated from Marine-Derived Fungi
In this section, a comprehensive summary of 320 structurally diverse meroterpenoids isolated from the culture extracts of marine-derived fungi over the period of January 2009 to December 2019 is presented. All the isolated compounds were classified according to their featured terpenoid part, i.e., hemiterpenes, monoterpenes, sesquiterpenes and diterpenes. The relevant biological and pharmacological activities of the reported compounds are provided wherever applicable.  (6), were obtained from the culture of the marine-derived fungus Acremonium persicinum, which was isolated from the marine sponge Anomoianthella rubra. None of the isolated compounds were assayed for any biological activity [22]. Mycophenolic-acid-based merohemiterpenes 7-17 ( Figure 1) were isolated from Penicillium bialowiezense, which was obtained from the soft coral Sarcophyton subviride. Compounds 7-17 exhibited an inhibitory activity against inosine-50-monophosphate dehydrogenase (IMPDH2) with IC 50 values ranging from 0.59 to 24.68 µM. These compounds were also assayed for the in vitro immunosuppressive activity against the proliferation of T-lymphocytes, and 7-9 exhibited IC 50 (Figure 2) were isolated from the sponge-derived fungus Arthrinium sp., obtained from the inner tissues of a marine sponge Sarcotragus muscarum collected off the coast of Southern Turkey. Both compounds did not show any significant in vitro cytotoxic activity against the Caco-2 (human epithelial colorectal adenocarcinoma) cell line [24]. A bicyclic merohemiterpene, acremine S (20) (Figure 2), was recently isolated from the marine-derived fungus Acremonium persicinum KUFA 1007 which was isolated from the marine sponge Mycale sp., collected from the coral reef in the Gulf of Thailand. Although 20 exhibited a weak inhibitory activity against acetylcholinesterase (AChE), its activity against butyrylcholinesterase (BuChE) was threefold higher than that of the positive control galantamine [25]. Acremines N (21), O (22), P (23) Q (24), R (25), spiroacremines A (26) and B (27) and 5-chlorospiroacremine (28) (Figure 2) were isolated from the marine-derived fungus Acremonium persicinum, obtained from a marine sponge Anomoianthella rubra. However, no bioactivity of the isolated compounds was investigated [22].
Austinol (165) (Figure 16), dehydroaustin (172) and 11α-acetoxyisoaustinone (189) ( Figure 18) were isolated from the culture extract of Penicillium citrinum, which was obtained from the mangrove Bruguiera sexangula var. rhynchopetala, collected in the South China Sea. Compounds 165, 172 and 189 showed selective antibacterial activity against five terrestrial and two marine pathogenic bacteria, particularly 165 displayed moderate activity against Staphylococcus epidermidis and S. aureus with MIC values of 10 µM. However, these compounds showed no cytotoxicity (IC 50 > 50 µM) against HeLa, MCF-7 and A549 cell lines [51]. Talaromytin (167) (Figure 16 (Figures 16 and 17).  Compounds 165, 169, 173, 174, 178, 181 and 185 exhibited insecticidal activity against a nematode Caenorhabditis elegans with EC 50 values ranging from 9.4 (± 1.0) to 38.2 (± 0.6) µg/mL [55]. Hwang et al. [56] reported the isolation of the previously reported austinoids, including 164, 172, 173, 175, 186, 5 S-isoaustinone (187) (Figure 18), 189, neoaustin (191) and austinoneol A (193) (Figure 18), from the culture extract of the marine-derived fungus Penicillium sp. FCH061, isolated from the underwater sediment collected off the coast of Chuja-do in Korea. It is worth mentioning that the stereostructures of these compounds in the reference are opposite to those described in this review. Asperaustins A (168) ( Figure 16) and B (190) ( Figure 18) were obtained, together with the previously described austinoids, i.e., 164, 172, precalidodehydroaustin (177), 186, 187, 193, from the culture extract of Aspergillus sp. ZYH026, isolated from superficial mycobiota of the brown alga Saccharina cichorioides f. sachalinensis, which was collected from the South China Sea [57] ( Figure 17). The absolute structures of 168, 177, 190 and 193 were established unambiguously by single-crystal X-ray analysis using CuKa radiation. All the isolated compounds, except 168, were assayed for AChE inhibitory activity but none exhibited significant activity [57].  austinoids including 165, 166,  169, 172, 173, 174, 188 and 191, from the marine-derived fungus Penicillium brasilianum WZXY-M122-9, isolated from a marine sponges collected from the South China Sea. None of the isolated compounds exhibited either cytotoxicity against A549, RAW264.7 (mouse monocyte/macrophage) and IEC-6 (rat small intestine epithelial) cell lines or antibacterial activity against Gram-positive bacteria S. aureus ATCC 29,213 and a clinically isolate Gram-negative bacteria Klebsiella pneumoniae 58AP) [58] (Figure 18). were determined by single-crystal X-ray analysis using CuKa radiation. Li et al. [60] described the isolation of two new analogues of berkeleyacetal (which were later reisolated by Hoang et al. [61] and named 22-deoxyminiolutelide B (201) ( Figure 20) and miniolutelide C (202) (Figure 19)), along with the previously reported 198, 200 and berkeleydione (203) (Figure 19), from Penicillium strain 303#, obtained from sea water from Zhanjiang Mangrove National Reserve in Guangdong Province, China. Compounds 201 and 202 displayed moderate cytotoxicity against MDA-MB-435, HepG2, HCT-116, and A549 cell lines [60]. Two previously unreported preaustinoid analogs, preaustinoids E (204) and F (205), were isolated together with the previously reported preaustinoid A2 (206) (Figure 19) and preaustinoid D (207) (Figure 20) from the underwater sediment-derived fungus Penicillium sp. FCH061. The relative stereochemistry of 204 and 205 was determined by nuclear overhauser effect spectroscopy (NOESY) correlations and compared with that of the previously reported compounds [56]. Zhang et al. [62] described the isolation of six new preaustinoid derivatives namely brasilianoids A (208), B (204), C (205), D (209), E (210) and F (211) (Figure 19), together with the previously reported 206 and 207, from a marine-derived fungus P. brasilianum WZXY-m122-9, isolated from an unidentified sponge. Surprisingly, the structures of brasilianoids B (204) and C (205) are found to be the same as those of preaustinoids E (204) and F (205), previously isolated by Wang et al. [56], although the stereostructures of 204 and 205 in the original article [56] are opposite to those of brasilianoid B (204) and C (205). Compound 208 significantly stimulated filaggrin and caspase-14 expressions in HaCaT (human keratinocyte) cells in a dose-dependent manner. Since filaggrin is a key natural moisturizing factor that maintains the ability to regulate the skin moisture barrier, 208 could be a potential cosmeceutical for skin moisturizer in the cosmetic industry. Moreover  (Figure 20), from the culture extract of the marine-derived fungus Penicillium ubiquetum MMS330, isolated from a sample of the blue mussel Mytilus edulis, collected at Port Giraud on the Loire estuary in France. All the isolated compounds, except 222, were evaluated for their cytotoxicity against KB (keratin-forming tumour) and MCF-7 cell lines, however, neither of them exhibited significant activity. The previously undescribed preaustinoid derivatives, preaustinoids A6 (229) and A7 (220) (Figure 20) were reported, together with the previously described 206, 224, and preaustinoid A3 (230) (Figure 20), from the marine-derived fungus Penicillium sp. SF-5497. Compound 224 and 229 inhibited PTP1B (a member of the protein tyrosine phosphatase (superfamily) activity in a dose-dependent manner with IC 50 values of 58.4 and 17.6 µM, respectively. Mechanistic study revealed that 201 inhibited PTP1B in a noncompetitive manner and preferentially bound to the free enzyme rather than to the enzyme-substrate complex [64]. Wen et al. [57] reported the isolation of a preaustinoid, named asperaustin C, from the algicolous fungus Aspergillus sp. ZYH026, which was claimed to be a new compound. The structure of asperaustin C, whose structure and absolute configurations of its stereogenic carbons were confirmed by X-ray analysis using Cu Kα radiation, was found to be the same as that of the previously described brasilianoid B (204). As the absolute configurations of the stereogenic carbons of preaustinoids E (204) and F (205), preaustnoid A2 (206) and preaustinoid D (207), reported by Hwang et al. [56], were determined by comparison with those described before the revision of the absolute configurations by Zhang et al. [62] and Wen et al. [57], the stereostructures of these compounds are opposite to those presented in this review.  Terretonins E (231) and F (232) ( Figure 21) were isolated from the culture extract of the marine derived-fungus Aspergillus insuetus, isolated from the marine sponge Petrosia ficiformis which was collected in the Mediterranean Sea. Compounds 231 and 232 inhibited NADH oxidase activity (in beef heart submitochondrial particles) with IC 50 values of 3.90 ± 0.4 and 2.97 ± 1.2 µM, respectively [65]. The culture extract of the marine sponge-associated fungus Aspergillus sp. OPMF00272 furnished terretonin G (233) and terretonin (234) (Figure 21). Compound 233 (20 mg per 6 mm disk) exhibited antibacterial activity against Gram-positive bacteria (S. aureus FDA209P, B. subtillis PCI219 and M. luteus ATCC9341), but not against Gram-negative bacteria (P. aeruginosa IFO12689 and E. coli JM109) and yeast (C. albicans ATCC64548 and S. cerevisiae S288c) [66]. Chemical examination of the endophytic fungus Aspergillus terreus EN-539, obtained from the fresh tissue of the marine red alga Laurencia okamurai which was collected from the coast of Qingdao, China, led to the isolation of the previously unreported terretonin analogue, aperterpene O (235), together with the previously described terretonins A (236) ( Figure 21) and G (233) [67]. Compound 233 exhibited antimicrobial activity against M. luteus and S. aureus with MIC values of 32 and 8 µg/mL, respectively [67]. A new terretonin analog terretonin O (237) was isolated, together with the previously reported terretonins M (238) and N (239) (Figure 21) from the culture extract of Aspergillus terreus LGO13, obtained from a sediment sample collected from sewage water containing heavy metals. Compound 237 displayed weak antimicrobial activity against P. aeruginosa and S. aureus [68]. The previously unreported terretonin D1 (240) (Figure 21) was isolated, together with the previously described 234, 236 and terretonin D (241) (Figure 21), from the marine-derived fungus Aspergillus terreus ML-44, obtained from the fresh gut of pacific oyster. All the isolated compounds displayed a weak inhibition of NO production in the LPS-stimulated RAW264.7 macrophages [69]. It is interesting to note that only the relative configurations of the structures of 231-233 were determined. On the contrary, the absolute configurations of the stereogenic carbons of 237-241 were established, with absolute confidence, by X-ray analysis using CuKα radiation with good Flack parameter. Therefore, it is possible that the structures of 231-233 are the enantiomeric form of their correct structures.  (Figure 22), were isolated from the marine-derived fungus Penicillium sp. YPGA11, obtained from the deep-sea water at a depth of 4500 m in the Yap Trench, West Pacific Ocean. Compounds 242-249 exhibited inhibitory activity against NO production in LPS-activated RAW 264.7 macrophages with inhibition rates ranging from 60% to 90% at 50 µM, but decreased sharply at 25 µM. Since these compounds were also cytotoxic to the RAW 264.7 cells (45-65% inhibition at 50 µM), it is believed that their inhibition of NO production was attributed to cell death [70]. Chemical examination of the algicolous fungus Aspergillus terreus EN-539 led to the isolation of another andrastin derivative named aperterpene N (250) (Figure 22) [67]. Compound 250 displayed the in vitro inhibitory activity against the influenza neuraminidase with an IC 50 value of 18.0 nM [67]. Andrastone A (251), 16-epi-citreohybriddione (252) and citreohybriddione A (253) (Figure 22) were recently isolated from the marine-derived fungus P. allii-sativi, isolated from the deep-sea water of the western Pacific. All the isolated compounds were evaluated for their antiproliferative effects against HepG2, A549, BIU-87 (urinary bladder), BEL-7402, ECA-109 (esophageal squamous carcinoma), HelaS3 (cervix), and PANC-1 (prancreatic) human tumour cell lines; however, only 251 displayed significant activity, with selective effect against HepG2 tumour cells with an IC 50 = 7.8 µM. Compound 251 also significantly increased caspase-3 and caspase-8 activities, but exhibited almost no effect on caspase-9. Moreover, this compound was found to increase the reporter transcriptional activation of RXRα (retinoid X receptor α) while reducing the transactivity of RXRα induced by 9-cis-retinoic acid in the luciferase reporter gene assay [71]. It is interesting to note that the absolute configurations of the stereogenic carbons of the sesquiterpene skeleton of 250, i.e., C-5, C-8, C-9, C-10, C-13 and C-14 are opposite to those of 242-249 and 252-253. Biogenetically, this is improbable. Even though the absolute structures of 249 [70] and 253 [71] were established by X-ray analysis, and those of 242-244 [70], 251 and 252 [71] were determined by comparison of the experimental and calculated (using TDDFT) ECD spectra, the parameters of the methods of determination of the configurations such as the flack parameter (in X-ray crystallography) and the precise wave length in the ECD curves of the experimental and calculated spectra should be duly taken into consideration.  The highly oxygenated merosesquiterpenes containing a rearranged drimane linked to an isochromone moiety, aspertetranones A-D (255-258) (Figure 23), were isolated from the culture extract of the endophytic fungus Aspergillus sp. ZL0-1b14 obtained from the marine green algal species of the genus Enteromorpha, which was collected from Jinjiang Dongshi salt pan in Fujian Province, China. Compounds 255-258 displayed weak inhibitory activities against TNF-α and NO production by LPS-stimulated RAW264.7 macrophages [73].

Meroditerpenoids
Naturally occurring meroditerpenoids can be categorized into three main classes: (i)-diterpenes combined with 3,5-dimethylorsellinic acid, (ii)-diterpenes combined with polyketides, and (iii)diterpenes combined with indole derivatives.  [74]. BACE1 was identified as being responsible for the formation of amyloid beta (Aβ) which is a highly aggregatory peptide segment of the membrane-associated amyloid precursor protein. Since Aβ aggregate is one of the targets for the drug discovery for Alzheimer's disease (AD), 259 and 260 could be an interesting model for a development of AD's drugs.  (Figure 25) was isolated from the culture extract of the marine sponge-associated fungus N. tsunodae KUFC 9213 which was obtained from the marine sponge Aka coralliphaga, collected from the Gulf of Thailand. Compounds 262 and 264 were examined for their cytotoxic activity against MCF-7, NCI-H460 and A375-C5 (melanoma) cell lines, using the protein binding dye sulforhodamine B (SRB) method. Compound 262 displayed potent growth inhibitory activity against the three cell lines, with GI 50 values of 13.6 ± 0.9, 11.6 ± 1.5 and 10.2 ± 1.2 µM, respectively, whereas 263 exhibited no activity at a concentration as high as 150 µM. Compound 264 also showed strong growth inhibitory activity against the same tumour cell lines, although less than that of 262, with GI 50 [76]. A new aszonapyrone analogue, sartorypyrone C (268) (Figure 25), was isolated from the culture extract of the marine-derived fungus N. paulistensis KUFC 7897, obtained from the marine sponge Chondrilla australiensis, collected from the Gulf of Thailand [76]. Sartorenol (265) (Figure 25), a triclyclic meroditerpene, was isolated, together with 262 and chevalone B (266) (Figure 25), from the algicolous fungus N. takakii KUFC 7898, obtained from the marine macroalga Amphiroa sp., collected in the Gulf of Thailand. Compound 265 showed no antibacterial activity against the above-mentioned four reference strains and environmental multidrug-resistant isolates [77]. Chemical examination of the marine-derived fungus N. siamensis, isolated from the sea fan Rumphella sp. which was collected from the Andaman Sea of Thailand, led to the isolation of chevalone C (267) (Figure 25). Compound 267 exhibited moderate cytotoxicity against three tumour cell lines including colon HCT-116, liver HepG2 and melanoma A375 with IC 50 values ranging from 24 to 153 µM [78]. Compound 266 was also recently reported from Aspergillus sp. H30 which was isolated from a sea cucumber Cucumaria japonica, collected from the South China Sea. Although 266 displayed weak antimicrobial activity against C. albicans SC5314 and methicillin-resistant S. aureus (MRSA), it exhibited cytotoxic activity against BC1 (lymphoma), KB, and NCI-H187 tumour cell lines [79].    (Figure 27). Compounds 297-306 were tested for their antibacterial activity against several human-, aqua-, and plant-pathogenic microbes; however, the tested compounds displayed antimicrobial activity in micromolar range against P. aeruginosa, E. coli, Vibrio parahaemolyticus and V. alginolyticus [83]. Penicindopene A (307) (Figure 28), an indole-bicylic diterpene, was isolated from the culture extract of Penicillium sp. YPCMAC1, obtained from the deep-sea water at a depth of 4500 m of the Yap Trench in the West Pacific Ocean. Compound 307 displayed a moderate antitumour activity against A549 and HeLa cell lines with IC 50 values of 15.2 and 20.5 µM, respectively [84]. Two indole-tricyclic diterpenes, penijanthines C (308) and D (309) (Figure 28), were reported together with 305 and 7-hydroxy-13-dehydroxypaxilline (310) (Figure 28), from the marine-derived fungus Penicillium janthinellum, which was isolated from a marine sediment collected from the Bohai Sea. Compounds 305, 308, 309 and 310 displayed significant growth inhibitory activity against Gram-negative halophilic pathogenic bacteria V. anguillarum, V. parahemolyticus, and V. alginolyticus with MIC values ranging from 3.1 to 50.0 µM [85] (Figure 28).

Conclusions and Prospects
The marine world represents the largest and most diverse ecosystem on earth. Since 1950 s, marine natural products chemists have raised the prospects of marine natural products (MNPs) as a great potential and renewable pipelines for compounds of a huge interest in pharmaceutical, nutraceutical and cosmetic industries. Marine microorganisms have become increasingly attractive as sources of compounds with unique structural features and unprecedented pharmacological activities. Marine-derived fungi represent an important source of MNPs due to their variable habitats from the tropics to the polar regions, from the surface to the seafloor and even at such extreme temperature and pressure as in a hydrothermal vent. Moreover, marine-derived fungi are also a prolific source of secondary metabolites capable of synthesizing a myriad of chemical classes of compounds. One of the most interesting classes of fungal secondary metabolites is meroterpenoids. According to our literature search over the period of January 2009 to the end of December 2019, 320 marine meroterpenoids have been reported from a myriad of marine-derived fungi from different habitats, many of which possess unique structural features and undescribed biological and pharmacological activities. At present, natural products from marine-derived fungi have not yet attained the status of the compounds produced by other marine organisms in the pharmaceutical industry. However, this is a question of time since many compounds produced by terrestrial fungi have been approved and successfully marketed as antibiotics, anticholesterolemic, among others. Moreover, many compounds have been successfully explored as cosmeceuticals and nutricosmetics whose market is in a marked expansion. Thus, the contribution of MNPs is undoubtedly vital not only for the pharmaceutical industry but also for other health industries, as well as for the preservation of the marine environment. Marine fungi are undoubtedly an important reservoir of a hidden treasure awaiting to be explored. With a rapid advancement of culture techniques, genome mining to uncover biosynthetic gene clusters, extraction processes and molecular techniques for bioassays, marine-derived fungi could become a great potential to provide valuable compounds as leads for drug development to combat many diseases, to maintain our healthy appearance and even for molecular tools to unlock the mechanisms of many rare and incurable diseases. With modern biotechnological processes, marine-derived fungi can be a huge renewable and untapped source of bioactive natural products while keeping the marine environment intact.