Austin-Type Meroterpenoids from Fungi Reported in the Last Five Decades: A Review

Austin was first isolated as a novel polyisoprenoid mycotoxin from Aspergillus ustus in 1976. Subsequently, some new austin-type meroterpenoids (ATMTs) have been continually found. This review attempts to give a comprehensive summary of progress on the isolation, chemical structural features, biological activities, and fungal biodiversity of 104 novel ATMTs from 5 genera of terrestrial- and marine-derived fungi reported from October 1976 to January 2023. The genera of Penicillium and Aspergillus are the two dominant producers, producing 63.5% and 30.8% of ATMTs, respectively. Moreover, about 26.9% of ATMTs display various pronounced bioactivities, including insecticidal, anti-inflammatory, cytotoxicity, antibacterial, and PTP1B inhibitory activities. The chemical diversity and potential activities of these novel fungal ATMTs are reviewed for a better understanding, and a relevant summary focusing on the source fungi and their taxonomy is provided to shed light on the future development and research of austin-type meroterpenoids.


Introduction
Microbial secondary metabolites differ from primary nutrients in that they are not essential for growth, but they play a vital role in the survival and adaptation of microbes in nature [1].Fungi attract much attention from chemists and biologists due to the production of secondary metabolites with diverse structural skeletons and interesting bioactivities.Austin-type meroterpenoids (ATMTs) are a family of hybrid natural products with high diversity of intriguing scaffolds, but only 104 ATMTs have been characterized, which are a relatively rare branch of the terpenoid family and have often been isolated from fungi, especially from the genera Penicillium and Aspergillus [2][3][4][5][6][7][8].The naturally occurring meroterpenoids derived from 3,5-dimethylorsellinic acid (DMOA) incorporated with farnesyl pyrophosphate (FPP) are the most common subclass [9][10][11][12][13], of which ATMTs are a class of special and important constituents.Based on the number of rings of the initial meroterpenoid in their biosynthesis pathway, ATMTs are classified into four categories, including tetracyclic, pentacyclic, hexacyclic, and heptacyclic systems.Interestingly, different types of ATMTs display broad and impressive biological activity [14][15][16], including insecticidal, antiphlogistic, antimicrobial, and antineoplastic effects, etc.
To date, no individual and comprehensive identification of the chemical structures of ATMTs has been reported.Therefore, this review was prepared to provide an overall coverage of the chemical constituents of the ATMTs reported in the last five decades (from October 1976 to January 2023) originating from fungi according to a classification of their chemical skeletons (two databases were used for the search: SciFinder and Web of Science).This review will provide information on the isolation, chemical structural features, biological activities, and fungal biodiversity of ATMTs, which will facilitate further research and exploitation of these structures.In this review, the biosynthesis pathways of these ATMTs are not discussed as they have been extensively reviewed by Wang, Abe, and Brakhage [17][18][19][20].
Preaustinoid D (34) was isolated from the endophytic fungus Penicillium sp.T2-8 associated with Gastrodia elata and showed moderate activity against Candida albicans with an MIC of 128 µg/mL [33].Bioinformatics analysis of the gene clusters in association with the qRT-PCR detection revealed the amplification of two key genes (clusters A and B) for the sponge-associated fungus Penicillium brasilianum WZXY-m122-9.Chromatographic separation of the EtOAc extract from the large-scale fermentation of this fungal strain resulted in the isolation of two new ATMTs, namely, brasilianoids D and E (35)(36) [34].The marine-derived fungal isolate Penicillium sp.SF-5497 resulted in the isolation of two new meroterpenoids, named preaustinoid A6 (37) and preaustinoid A7 (38); furthermore, preaustinoid A6 (37) (Figure 4) inhibited protein tyrosine phosphatase-1B (PTP1B) in a noncompetitive manner, with a K i value of 17.0 µM [35].

Hexacyclic Systems-Rings A and B Are Bicyclic Fused
Dhilirolide L (98) and dhilirolide N (99) were isolated from P. purpurogenum, which was obtained from rotting fruit of a Averrhoa bilimbi tree growing in Sri Lanka.Compound 98 showed significant feeding inhibition and sublethal developmental disruption in the cabbage looper Trichoplusiani with a DC 50 value of 5.9 µg/cm 2 [12] (Figure 10).Brasilianoid F (100) was obtained from the large-scale fermentation of P. brasilianum WZXY-m122-9 [34].

Heptacyclic Systems Austin-Type Meroterpenoids Heptacyclic Systems-Rings A and B Are Bicyclic Fused
Dhilirolides A-C (101-103) were isolated from laboratory cultures of the fruit-infecting fungus P. purpurogenum collected in Sri Lanka [13] (Figure 11).The fungus P. purpurogenum obtained from rotting fruit of a A. bilimbi tree growing in Sri Lanka yielded one new austin-type meroterpenoid, named dhilirolide J (104) [25].

Comprehensive Overview and Conclusions
To the best of our knowledge, investigations on the chemical constituents of ATMTs in the last five decades have reported fungal biodiversity, and total of 104 novel ATMTs from 5 genera of terrestrial-and marine-derived fungi have been reported in 40 research papers published from October 1976 to January 2023 (Table 1, Figures 12-15).In total, 37% of the compounds are categorized as the tetracyclic type (up to 38 compounds) followed by pentacyclic (32%, 33), hexacyclic (27%, 29), and heptacyclic (4%, 4) types (Figure 12).This review summarizes the source, chemistry, and biological activities of the novel ATMTs.The majority of the ATMTs have been isolated since 2010, accounting for about 80% of all reported ATMTs (83/104) and 75% of the published articles (Figure 13).The increase was largely due to improvements in isolation procedures.All of the published ATMTs that have been isolated and identified in various filamentous fungi, most of them were produced by Penicillium (62%) and Aspergillus (30%), representing more than 90% of the secondary metabolites reported (Figure 14).The remaining (about 10%) were produced by Pestalotiopsis (2%), Eupenicillium (2%), and Emericella (4%).
(     In terms of the source distribution of compounds, one surprising discovery was that all of these novel ATMTs were isolated from fungi, including 39% terrestrial-and 43% marine-derived endophytic fungi, much more than those produced by unknown sources (18%) (Figure 15).The results showed that these kinds of ATMTs are a very special class of fungi metabolites, and more chemical ecological research needs to be carried out.Overall, this review provides a comprehensive overview of the diverse chemical structures and bioactive properties of 104 new ATMTs that have been isolated from fungi in the last five decades.Nearly 26% of the ATMTs showed bioactivities.About 7% exhibited inhibitory effects on NO production.Interestingly, about 9% ATMTs have been showed to possess selective insecticidal activity, about 2% exhibit antimicrobial activity, and 8% demonstrate other activities, including cytotoxicity and antityrosinase activity (Figure 16), revealing the untapped potential of activity ATMTs in pesticides and medicinal applications.However, for most of the isolated ATMTs, the lack of activity analysis and pharmacodynamic evaluation limit their application.In summary, a total 104 of ATMTs have been isolated and characterized to date since the initial discovery of austin-type meroterpenoids in 1976.Although ATMTs have a distinct a unique chemical skeleton and potential biological activities, the unavailability of large amounts of natural austin-type meroterpenoids and purification challenges due to their structural complexity hindering efficient chemical synthesis have hindered further research.Most of the ATMTs with novel skeletons and biological activities have been discovered in recent years.The further development and application of these compounds is important, and the identification of promising lead compounds for the development of drugs is a critical future direction of study.

Figure 13 .
Figure 13.The production of austin-type meroterpenoids and published articles per decade.

Table 1 .
The compounds isolated from fungi (the structures of the compounds are illustrated in Figures1-11).
a Not reported.bNoactivity reported in the reference.