Anti-Alzheimer’s Natural Products Derived from Plant Endophytic Fungi

Abstract

Alzheimer’s is the most common cause of dementia worldwide and seriously affects patients’ daily tasks. Plant endophytic fungi are known for providing novel and unique secondary metabolites with diverse activities. This review focuses primarily on the published research regarding anti-Alzheimer’s natural products derived from endophytic fungi between 2002 and 2022. Following a thorough review of the literature, 468 compounds with anti-Alzheimer’s-related activities are reviewed and classified based on their structural skeletons, primarily including alkaloids, peptides, polyketides, terpenoids, and sterides. The classification, occurrences, and bioactivities of these natural products from endophytic fungi are summarized in detail. Our results provide a reference on endophytic fungi natural products that may assist in the development of new anti-Alzheimer’s compounds.

1. Introduction

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder and the most common cause of dementia worldwide that affects memory, thinking, and behavior and even interferes with daily tasks. The abnormal accumulation of beta-amyloid and phosphorylated tau proteins and nerve cell degeneration are deemed to play key roles in Alzheimer’s disease [1,2]. According to the latest WHO report, the number of people suffering from dementia worldwide in 2010 was about 35.6 million, while the figure could be triple this by 2050 [1]. Age is the biggest risk factor for Alzheimer’s dementia, which dramatically increases the incidence and death rate of Alzheimer’s dementia and contributes to a heavy burden on families and society. The incidence of dementia was 5.0–13.1% for people over 65 years old, while this number increased to 33.2% as the age rose to over 85 years of age, and the death rate increased by 33–51% for people over 65 years of age and by 78% for people aged 80 and older [3]. Only a few therapeutic agents have been made clinically available for this disease, such as memantine, donepezil, rivastigmine, tacine, galantamine, and aducanumab [4,5,6]. These drugs can relieve AD-related symptoms for mild cognitive impairment, but are incapable of preventing disease progression to obtain ideal treatment effects [7]. Thus, it is critical to develop new treatments for AD to prevent and delay the progression of the disease, improve cognition, and reduce the behavioral disorders of patients with AD.

Endophytic fungi were first identified in plants in 1809 [8]. They are microorganisms that reside in the tissues of healthy plants for part of or all of their life cycle without causing apparent infection in the host plant. Some endophytes provide new bioactive compounds with unique structures containing alkaloids, phenols, lactones, quinones, terpenoids, steroids, and other compounds. These isolated metabolites display antibiotic, antioxidant, anti-inflammatory, antiviral, and anti-Alzheimer’s properties, among others [9,10,11,12].

In this review, a comprehensive survey of approximately 468 compounds with anti-Alzheimer’s-related activities derived from endophytic fungi from 2002 to 2022 is performed. These compounds are classified by their structure skeleton and mainly include alkaloids, peptides, polyketides, terpenoids, and sterides. The most investigated activities of these metabolites are the inhibition of acetylcholinesterase (AChE), butyrylcholinesterase (BChE), neuroprotection, β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) inhibition, and their antioxidant activities. So far, the secondary metabolites of plant endophytic fungi with anti-Alzheimer’s activities have not been summarized. This review mainly focuses on the classification, occurrences, and bioactivites of the secondary metabolites derived from endophytic fungi.

2. Bioactive Compounds from Plant Endophytic Fungi

2.1. Alkaloids

2.1.1. Cytochalasans

The chemical study of endophyte Xylaria sp. collected from the leaves of Casearia sylvestris showed cytochalasins B–D (1–3) (Figure 1). Compounds 1 and 2 showed strong anti-AChE activities at 60 µg [13]. Research on Aspergillus terreus obtained from the stems of Artemisia annua afforded the four known cytochalasans cytochalasin E (4), 5,6-dehydro-7-hydroxy cytochalasin E (5), Δ^6,12-isomer of 5 (6), and rosellichalasin (7). Compounds 4–7 showed weak anti-AChE activities with IC₅₀ from 110.9 to 176.0 μM [14]. Cytochalasins J (8) and H (9) were identified from endophyte Phomopsis sp., which was isolated from Senna spectabilis (Fabaceae). Compound 9 demonstrated AChE inhibition in vitro at a minimum quantity of 25.0 µg [15].

Two heterocycle-fused cytochalasan homodimers, bisaspochalasins D (10) and E (11), along with aspochalasins D (12) and B (13), were identified from an endophytic Aspergillus flavipes associated with the stems of Hevea brasiliensis. Among them, compound 10 alone exhibited neurotrophic activities that could accelerate neurite growth with a differentiation rate of 12.52% for rat pheochromocytoma cells (PC12) at 10 μM [16].

Seven compounds containing chaetoglobosins A (14), B (15), E (16), F (17), and F_ex (18) as well as penochalasins F (19) and G (20) were separated from Chaetomium globosum, an endophytic fungus associated with the seeds of Panax notoginseng. Compound 14 showed negligible anti-AChE activity with an inhibition ratio of less than 10% at 50 μM. None of them showed 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical-scavenging activity with half effective concentration (EC₅₀) greater than 100 ug/mL [17].

A total of six 10-indolyl-cytochalasans (16–18), cytoglobosin A (21), penocha-lasin C (22), and isochaetoglobosin D (23), were collected from Chaetomiun globosum WQ in Imperata cylindrical, and 9 and 18-methoxycytochalasin J (24) were identified from Phomopsis sp. IFB-E060 in Vatica mangachapoi. With the exception of 22, these compounds showed scavenging DPPH activity with an EC₅₀ between 0.002 ± 0.001 and 1.068 ± 0.350 mM, scavenging ABTS activity with an EC₅₀ between 0.002 ± 0.001 and 1.052 ± 0.357 mM, strong inhibiting activity of hydrogen peroxide (H₂O₂)-mediated PC12 cell damage with an EC₅₀ between 0.003 ± 0.0003 and 0.240 ± 0.236 µM, and inhibiting N-methyl-4-phenylpyridinium iodide (MPP⁺) induced PC12 cell damage activity with an EC₅₀ between 0.009 ± 0.007 and 6.100 ± 0.007 µM [18].

Chemical research on mangrove endophyte Westerdykella nigra collected from the roots of Avicennia marina (Forssk.) Vierh. resulted in the isolation of westalsan (25), phomacin B (26), and 19-hydroxy-19,20-dihydrophomacin C (27), which showed apparent AChE inhibition with IC₅₀s of 0.056 ± 0.003, 0.088 ± 0.005, and 0.140 ± 0.007 μM, respectively [19].

2.1.2. Diketopiperazine Derivatives

Detailed chemical research on an endophytic fungus, Acrostalagmus luteoalbus TK-43, collected from marine green alga Codium fragile, led to the identification of three pairs of indolediketopiperazine enantiomers (acrozine A (28/29), acrozine B (30/31), and acrozine C (32/33)), four new congeners (acrozines D–G, 34–37), and six known analogs (pseudellone D (38), lasiodipine E (39), chetoseminudin C (40), chetoseminudin B (41), T988 C (42), and T988D (43)) (Figure 2). Compounds 28–37 possessed an unusual methoxy site in N-2, which was scarcely reported in indolediketopiperazine alkaloids. The evaluation of compounds 28–37 for anti-AChE activity revealed that compound 28 displayed better inhibition with an IC₅₀ of 2.3 µM than did 29 (IC₅₀ = 13.8 µM). Compounds 30–33 demonstrated moderate and weak AChE inhibitory activities with IC₅₀ values in the range of 49.2 to 160.6 µM [20]. The IC₅₀ for AChE inhibition by compound 36 was 8.4 μM. Other compounds showed weak activities at 32 μM [21].

Acetylapoaranotin (44) was identified from the liquid fermentation of Aspergillus terreus associated with the stems of Artemisia annua. The IC₅₀ of compound 44 for anti-AChE activity was 127.4 μM [14].

Three known alkaloids, cyclotryprostatin B (45), fumitremorgin B (46), and fumitremorgin A (47), were isolated from the endophyte Neosartorya fischeri JS0553 of Glehnia littoralis. None of these alkaloids showed obvious neuroprotection against glutamate-mediated HT22 cell injury at 20 μM [22].

Fumitremorgin C (48), brevianamide F (49), spirotryprostatin A (50), 6-methoxyspirotryprostatin B (51), 3-dehydroxymethylbisdethio-3,10a-bis(methylthio)gliotoxin (52), bisdethiobis(methylthio)gliotoxin (53), and didehydrobisdethiobis(methylthio)gliotoxin (54) were collected from endophyte Talaromyces sp. LGT-2 associated with Tripterygium wilfordi. Compound 53 showed weaker anti-AChE activity [23].

A chemical study of Nigrospora camelliae-sinensis S30 collected from mangrove Lumnitzera littorea afforded two new 2,5-diketopiperazine derivatives, nigrosporaamides A and B (55, 56), and seven known analogs (57–63): cyclo-(L-Pro-L-Phe) (57), cyclo[L-(4-hydroxyprolinyl)-L-Leu] (58), cyclo-(L-Val-L-Pro) (59), cyclo-(L-Leu-L-Pro) (60), cyclo-(R-Leu-R-Pro) (61), cyclo-(L-Ile-L-Pro) (62), and cyclo-(4-methyl-R-Pro-S-Nva) (63). None showed significant neuroprotection against H₂O₂-mediated cytotoxicity for HT22 cells at 100 μM [24]. In addition, compound 59 was also discovered in potato dextrose broth fermentation cultures of Penicillium sp.1, an endophytic fungi living in the leaves of Alibertia macrophylla (Rubiaceae), which exhibited potent AChE inhibition with a detection limit of 10.0 μg [25].

Diketopiperazines cyclo-(S-Pro-S-Tyr) (64) and cyclo-(S-Pro-S-Val) (65) were isolated from Colletotrichum gloeosporioides [26]. Cyclo(D)-Pro-(L)-Val (66), cyclo(D)-Pro-(D)-Tyr (67), cyclo(D)-Val-(D)-Tyr (68), cyclo(D)-Hyp-(L)-Ile (69), cyclo(D)-Pro-(D)-Leu (70), cyclo(D)-Pro-(L)-Ile (71), cyclo(D)-Pro-(L)-Phe (72), and cyclo(D)-Pro-(D)-Phe (73) were isolated from Colletotrichum crassipes [13]. Among them, compounds 64 and 65 exhibited moderate AChE inhibitory activities at 200 μg [26].

2.1.3. Indole Alkaloids

One new alkaloid, 16α-hydroxy-5N-acetylardeemin (74), together with two known compounds, 5N-acetylardeemin (75) and 15b-β-hydroxy-5N-acetylardeemin (76) (Figure 3), were identified from the liquid fermentation of the endophyte Aspergillus terreus of Artemisia annua. Compounds 74–76 displayed anti-AChE activities with IC₅₀ values of 58.3, 149.4, and 116.9 µM, respectively [14].

A chemical study of the endophytic fungus Colletotrichum gloeosporioides collected from the leaves of Michelia champaca led to the isolation of a new compound, 2-phenylethyl 1H-indol-3-yl-acetate (77), which exhibited moderate AChE inhibitory activity at 200 μg during a bioautography analysis [26].

A new macfortine alkaloid, chrysogenamide A (78), was identified from Penicillium chrysogenum No. 005, an endophyte from the root of Cistanche deserticola. Compound 78 showed no scavenging DPPH free radical activity at 100 µM, while it exhibited the inhibition of H₂O₂-mediated SH-SY5Y cell death by enhancing cell viability by 59.6% at 1 × 10⁻⁴ μM, suggesting that 78 exhibited a protective effect on neurocytes via oxidative stress-mediated cell death in SH-SY5Y cells rather than through antioxidant activity [27].

An investigation of the endophytic fungus Aspergillus fumigatus of Cynodon dactylon revealed two new alkaloids, 9-deacetylfumigaclavine C (79) and 9-deacetoxyfumigaclavine C (80), as well as the known compound fumigaclavine C (81). These isolates were practically inactive to induce the neurie outgrowth of PC12 [28].

Two known alkaloids, aszonalenin (82) and acetylaszonalenin (83), were identified from Neosartorya fischeri JS0553, an endophyte of Glehnia littorali. Neither showed obvious neuroprotection against glutamate-induced HT22 cell damage [22].

A new indole alkaloid, alternatine A (84), and two known indole alkaloids, 1H-indole-3-carboxylic acid (85) and indole-3-methylethanoate (86), were identified from Alternaria alternate collected from the leaves of Psidium littorale Raddi. The cell viabilities of 86 were prominently increased by 75.6 ± 4.2% and 84.8 ± 6.5% at 40 and 80 μM, respectively [29]. Compound 85 was also identified in Epicoccum nigrum and Penicillium brefeldianum F4a collected from the fresh leaves of Entada abyssinica Steud. ex A. Rich. Fabaceae and the roots of Houttuynia cordata, respectively. This compound exhibited weak scavenging activity with IC₅₀ = 88.97 µg/mL in the DPPH assay and EC₅₀ = 21.48 ± 0.88 µM in the ABTS assay [30,31].

Seven dimeric tryptophol-related alkaloids, colletotryptins A–D (87–90), E (91/92), and F (93), were separated from the solid fermentation of Colletotrichum sp. SC1355, an endophytic fungus collected from the healthy leaves of Cleistocalyx operculatus. Compounds 87–93 did not show AChE inhibitory activity [32].

2.1.4. Other Alkaloids

The chemical investigation of endophyte Colletotrichum gloeosporioides identified in the leaves of Michelia champaca revealed two known compounds, uracil (94) and 4-hydroxy-benzamide (95) (Figure 4), which exhibited moderate AChE inhibitory activities at 200 μg [26].

One new metabolite, α-pyridone derivative 3-hydroxy-2-methoxy-5-methylpyridin-2(1H)-one (96), was isolated from Botryosphaeria dothidea KJ-1, an endophytic fungus from the stems of Melia azedarach L. This compound showed low DPPH scavenging activity with a rate of 22.5% at 50 μM [33].

One known compound, 5-(40-Hydroxybenzyl) hydantoin (97), identified from Nigrospora camelliae-sinensis S30 associated with mangrove Lumnitzera littorea, was not found to exhibit obvious neuroprotective activity against H₂O₂-mediated cytotoxicity for HT22 cells [24].

Four new racemic mixtures of 4-quinolone alkaloids, (±)-oxypenicinolines A (98/99); B (100/101); C (102/103); and D (104/105), and two congeners, penicinolines F (106) and G (107), as well as seven known related compounds, 1,2,3,11b-tetrahydroquinolactacide (108/109); quinolactacide (110); penicinoline (111); methyl-penicinoline (112); penicinoline E (113); quinolonimide (114); and 4-oxo-1 and 4-dihydroquinoline-3-carboxamide (115), were collected from Penicillium steckii SCSIO 41025 (Trichocomaceae), a mangrove-derived endophyte of Avicennia marina (Forsk.) Vierh (Trichocomaceae). Only compounds 111 and 113 showed mild AChE inhibition with IC₅₀s of 87.3 and 68.5 μM, respectively [34].

Endophyte Ceriporia lacerate HS-ZJUT-C13A identified in the medicinal plant Huperzia serrata was chosen for transforming hupA in a liquid potato–dextrose medium. Five unusual alkaloids, huptremules A–D (116–119) and 8α,15α-epoxyhuperzine A (120), were obtained. Among them, 116–119 characterized irregular sesquiterpenoid–alkaloid structural hybrids, which combined the features of fungal metabolites and the substrate of hupA. These isolates displayed significant AChE inhibition with IC₅₀ within a range of 0.06 to 12.11 μM (positive control hupA with an IC₅₀ of 0.54 μM) [35].

Chemical research on Aspergillus terreus (No. GX7-3B), a mangrove endophytic fungus from a branch of Bruguiera gymnoihiza (Linn.) Savigny, resulted in the isolation of 8-O-methylbostrycoidin (121), which showed prominent AChE inhibition with IC₅₀ at 6.71 μM [36].

A study on the endophytic fungus Fusarium sp. HP-2 identified the compound lumichrome (122), which did not exhibit AChE inhibition at 50 μM [37].

An investigation of Phomopsis sp. xy21 related to leaves of the Thai Xylocarpus granatum isolated phomopsol A (123) with a matchless 3,4-dihydro-2H-indeno [1,2-b]pyridine 1-oxide group. The cell activities of 123 were 76% at 40.0 μM, which showed neuroprotection against corticosterone-mediated PC12 cell injury with a concentration-dependent effect within the scope of 5.0–40.0 μM [38].

Two known compounds, 14-norpseurotin (124) and pseurotin A (125), were identified from Aspergillus fumigatus related to a healthy stem of Cynodon dactyl. Compound 124 had stronger activity than did 125 in promoting neurite outgrowth at 10.0 µM for PC12 [28].

Chemical research on Neosartorya fischeri JS0553 associated with Glehnia littoralis produced two known alkaloids: fischerin (126) and pyripyropene A (127). The investigation of the mechanisms for glutamate-induced HT22 cell injury revealed that 126 could inhibit Ca²⁺ influx, ROS, and the phosphorylation of JNK, ERK, and p38 to exert conspicuous neuroprotection [22].

Three new alkaloids, penazaphilone E (128), isochromophilone VI (129), and peniazaphilone D (130), were identified from Penicillium sp. JVF17 related to Vitex rotundifolia. Compounds 128–130 have been proven to possess almost 100% neuroprotection at 25 μM. The mechanism of 128 regarding glutamate-mediated HT22 cell death involved restraining MAPKs phosphorylation and reducing the Bax/Bcl-2 ratio [39].

An investigation of Cochliobolus lunatus SCSIO41401 led to the isolation of the lipopeptide epimers sinulariapeptides A (131) and B (132), which displayed obvious AChE inhibition with IC₅₀s of 1.8 ± 0.12 and 1.3 ± 0.11 μM, respectively [40].

Research on the endophytic fungus Rhizopycnis vagum Nitaf22 revealed a novel alkaloid, rhizovagine A (133), which has an unprecedented 5/5/6/6/6 integrated pentacyclic skeleton. This compound was found to exhibit AChE inhibition with an IC₅₀ of 43.1 μM [41].

The study of Talaromyces sp. LGT-2 associated with Tripterygium wilfordi resulted in the identification of pseurotin A1 (134) and pseurotin A2 (135), which showed weaker anti-AChE activities [23].

2.2. Peptides

Beauvericin (136) (Figure 5) was collected from Aspergillus terreus (No. GX7-3B) from a branch of Bruguiera gymnoihiza (Linn.) Savigny. The IC₅₀ of this compound for AChE inhibition was 3.09 μM [36].

Colletotrichamides A−E (137–141) were identified from Colletotrichum gloeosporioides JS419, a fungus collected from Suaeda japonica Makino. Colletotrichamide C (139) displayed potent neuroprotection with 100% cell activity at 100 μM against glutamate-induced HT22 cell death [42].

The study of Bipolaris sorokiniana LK12 led to the isolation of BZR-cotoxin I (142) and BZR-cotoxin IV (143), which possessed mild anti-AChE, lipid peroxidation, and urease activities [43].

A chemical study of Cryptosporiopsis sp. identified cryptosporioptide (144), which possessed significant lipoxygenase inhibition with an IC₅₀ of 49.15 ± 0.17 µM [44].

2.3. Polyketides

2.3.1. Pyranones and Pyranyl Derivatives

Simple Pyranones

Four new prenylated asteltoxin analogs, avertoxins A–D (145–148), along with the known mycotoxin asteltoxin (149) (Figure 6) were obtained from Aspergillus versicolor Y10 associated with the leaves of Huperzia serrata. The IC₅₀ of avertoxin B (146) for AChE inhibition was 14.9 μM [45].

A study on Xylaria sp. HNWSW-2 collected from the stem of Xylocarpus granatum led to the isolation of astropyrone (150), which diaplayed weak anti-AChE activity with an inhibition rate of 10.4% at 50 µg/mL [46].

The investigation of Bipolaris sorokiniana LK12 identified in Rhazya stricta revealed the isolation of bipolarisenol (151), which showed obvious AChE inhibition with an IC₅₀ of 67.23 ± 5.12 µg/mL and also displayed mild lipid peroxidation inhibition with an IC₅₀ of 168.91 ± 4.23 µg/mL [47].

Pycnophorin (152) was collected from Botryosphaeria dothidea KJ-1, which presented as a weak DPPH scavenger with a scavenging rate of 22.5% at 50 μM [34].

A chemical study of Chaetomium globosum associated with the seeds of Panax notoginseng led to the isolation of chaetomugilins A (153) and D (154). Neither showed antioxidant activities with an EC₅₀ greater than 100 μg/mL in DPPH free radical scavenging [17].

Benzopyrones

Chromone derivatives hydroxylchromone (155) (Figure 7); 6-hydroxymethyleugenin (156); 6-methoxymethyleugenin (157); chaetoquadrin D (158); isoeugenitol (159) and isocoumarin congeners diaporthin (160); 8-hydroxy-6-methoxy-3-methylisocoumarin (161); and 6-methoxymellein (162) were isolated from Xylomelasma sp. Samif07 related to Salvia miltiorrhiza Bunge. Compound 160 alone displayed powerful antioxidant activity with an EC₅₀ of 15.1 µg/mL in hydroxyl radical scavenging [48].

I-6-hydroxymellein (163), 6,8-dihydroxy-3-(10R, 20R-dihydroxypropyl)-isocoumarin (164), 6-hydroxy-8-methoxy-3-methylisocoumarin (165), and de-O-methyldiaporthin (166) were collected from Phaeosphaeria sp. LF5 associated with the leaves of Huperzia serrata. The IC₅₀ value of compound 166 for AChE inhibition was 21.18 µM. Other compounds showed inactivity at 100 µM [49].

4-Hydroxymellein (167), 8-methoxymellein (168), and 5-hydroxymellein (169) were isolated from Penicillium sp.2 collected from the leaves of Alibertia macrophylla (Rubiaceae). This was the first time compounds 168 and 169 had been isolated from the genus Penicillium. These compounds demonstrated moderate to weak AChE inhibitory activities [25].

α-Pyrone derivatives (167, 170–181) containing 4-hydroxymellein (167), palmariol B (170), alternariol 9-methyl ether (171), botrallin (172), hyalodendriols A–C (173–175), rhizopycnin D (176), penicilliumolide D (177), TMC-264 (178), penicilliumolide B (179), alternariol (180), and graphislactone A (181) were obtained from Hyalodendriella sp. Ponipodef 12, an endophyte from the hybrid ‘N’va’ of Populus deltoides Marsh × P. nigra L. L. Compounds 170–172, 174, 178, and 179 exhibited moderate to weak activities for AChE inhibition with IC₅₀ values within the scope of 21.1 to 135.52 μg/mL. Other compounds were inactive with an IC₅₀ beyond 200 µM for anti-AChE activities [50,51].

Four known compounds, including graphislactone A (182), graphislactone A diacetate (183), botrallin (172), and botrallin diacetate (184), were isolated and identified from Microsphaeropsis olivacea obtained from Pilgerodendron uviferum (D. Don) Florin (“Cipres de las Guaitecas”). Compounds 182, 183, 172, and 184 showed strong to moderate AChE inhibitory activities with IC₅₀s of 8.1, 88, 6.1, and 27 µg/mL, respectively [52].

Five isocoumarins, monocerin (185); monocerin demethylated derivative (186); fusarentin 6,7-dimethyl ether (187); fusarentin 6-methyl ether (188); fusarentin derivative (189); and phthalide (190) were collected from the Colletotrichum sp. CRI535-02 of Piperornatum. The IC₅₀s of compounds 186 and 188 were 23.4 and 16.4 µM for DPPH inhibition and 52.6 and 4.3 µM for superoxide anion radical inhibition, respectively. Isocoumarins 185–187 showed excellent ORAC antioxidation with 10.8–14.4 ORAC units, and 190 displayed antioxidation with 2.4 units [53].

Penialidin A (191), penialidin F (192), and myxotrichin C (193) were identified from Penicillium brefeldianum F4a associated with the roots of H. cordata. Compounds 192 and 193 could scavenge DPPH with EC₅₀s of 28.42 ± 3.16 and 30.07 ± 2.83 µM, respectively. Compounds 191–193 had the strongest scavenging ABTS⁺ activity with EC₅₀s of 14.54 ± 0.46, 7.61 ± 0.46, and 14.96 ± 2.57 µM, respectively [31].

A detailed chemical study of Phomopsis sp. 33#., an endophytic fungus from Rhizophora stylosa, led to the discovery of four new compounds, phomopsichins A–D (194–197), and the known compound phomoxanthone A (198). Compounds 194–198 showed weak inhibitory activities against AChE with an inhibitory rate from 2.7% to 38.4% for a concentration of 250 µM and displayed weak scavenging DPPH activity with an inhibitory rate from 17.0% to 52% at 1 mM [54].

A new compound, (2R,3S)-pinobanksin-3-cinnamate (199), isolated from the endophytic fungus Penicillium sp. FJ-1 of Acanthus ilicifolius Linn, exhibited a potent neuroprotective effect on corticosterone-damaged PC12 cells [55].

Three novel aromatic polyketide dimers, bialternacin A (200), bialternacin E (201), and bialternacin F (202), featured as racemic mixtures, were identified from a plant endophytic Alternaria sp. associated with stem of Maianthemum bifolium. Compound 192 alone exhibited AChE inhibition with an IC₅₀ of 15.5 μM [56].

Pyranyl Derivatives

A chemical study of Penicillium sp. JVF17 associated with Vitex rotundifolia led to the isolation of nine azaphilone-type polyketides, peniaphilones A–I (203–208, 210–212), together with dechloroisochromophilone III (209) and isochromophilone V (213) (Figure 8). Compounds 205, 208, 209, and 213 showed neuroprotective effects against glutamate-induced HT22 cell injury within the scope of 25 μM and 100 μM [39].

Three new azaphilones, chermesinones A–C (214–216), were collected from Penicillium chermesinum (ZH4-E2) associated with the stem of Kandelia candel. None exhibited the inhibition of AChE (IC₅₀ > 100 μM) [57].

The chemical investigation of Saccharicola sp. isolated from Eugenia jambolana resulted in the identification of two compounds: 2,2-dimethyl-2H-chromene-6-carboxylic acid (217) and trans-3,4-dihydro-3,4-dihydroxy-anofinic acid (218). Compound 218 displayed huAChE-ICER and eeAChE-ICER inhibitory activities with IC₅₀s of 0.037 ± 0.01 and 0.026 ± 0.005 mg/mL, respectively [58].

2.3.2. Quinones

The chemical investigation of Colletotrichum sp. JS-0367 associated with the leaves of Morus alba (mulberry) led to the identification of the new compound 1,3-dihydroxy-2,8-dimethoxy-6-methylanthraquinone (219) and the three known compounds 1-hydroxy-2,3,8-trimethoxy-6-methylanthraquinone (220), 1,2-dihydroxy-3,8-dimethoxy-6-methylanthraquinone (221), and evariquinone (222) (Figure 9). Compound 222 inhibited intracellular ROS aggregation, Ca²⁺ influx, MAPKs phosphorylation, and apoptotic cell death to exert potent protection against glutamate-mediated HT22 cell death [59].

Quinizarin (223) identified from Epicoccum nigrum, an endophyte from the fresh leaves of Entada abyssinica Steud. ex A. Rich., Fabaceae, exhibited significant ABTS and DPPH scavenging activities with IC₅₀s of 10.86 and 11.36 µg/mL, respectively [30].

A chemical study of the Chaetomium sp. YMF432 of Huperzia serrata (Thunb. ex Murray) Trev led to the discovery of known compounds 1-omethylemodin (224), 5-methoxy-2-methyl-3-tricosyl-1,4-benzoquinone (225), and isosclerone (226), which were identified in this fungus for the first time. Compounds 224 and 225 displayed mild AChE inhibition with IC₅₀s of 37.7 ± 1.5 and 37.0 ± 2.9 μM, respectively, while compound 226 was inactive for anti-AChE activity with an inhibition rate of less than 10% at 100 μg/mL [60]. In addition, isosclerone (226) was also identified from Alternaria alternate collected from the leaves of Psidium littorale Raddi., which showed neuroprotective activities for glutamate-injured PC12 cells by significantly improving cell viabilities with values ranging from 65.9 ± 3.9% to 74.6 ± 4.0% after treatment with the compound at 20, 40, and 80 μM [29].

Research on Aspergillus terreus (No. GX7-3B) related to a branch of Bruguiera gymnoihiza (Linn.) revealed the identification of an unusual thiophene, 8-hydroxy-2-[1-hydroxyethyl]-5,7-dimethoxynaphtho[2,3-b] thiophene-4,9-dione (227), as well as anhydrojavanicin (228), 8-O-methyljavanicin (229), botryosphaerone D (230), and 6-ethyl-5-hydroxy-3,7-dimethoxynaphthoquinone (231). The IC₅₀ of 228 for anti-AChE activity was 2.01 μM [36].

An investigation into Fusarium sp. HP-2 from “Qi-Nan” agarwood found two new naphthalenone analogs: 3-demethoxyl-fusarnaphthoquinone B (232) and (2S,3S,4S)-8-dehydroxy-8-methoxyl-dihydronaphthalenone (233). The inhibition ratio of 233 against AChE was 11.9% at 50 μM [37].

A detailed chemical study on endophyte Talaromyces islandicus EN-501 associated with red alga Laurencia okamurai led to the isolation of 8-hydroxyconiothyrinone B (234), 8,11-dihy-droxyconiothyrinone B (235), 4R,8-dihydroxyconiothyrinone B (236), 4S,8-dihydroxyconiothyrinone B (237), and 4S,8-dihydroxy-10-O-methyldendryol E (238). Compounds 234−238 exhibited antioxidant activities with IC₅₀ = 12, 31, 42, 52, and 30 μM against DPPH and IC₅₀ = 8.3, 19, 34, 31, and 24 μM against ABTS, respectively [61].

5-Methoxy-2-methyl-3-pentacosylcyclohexa-2,5-diene-1,4-dione (239) identified from the Colletotrichum sp. F168 of the plant Huperzia serrata Trev displayed negligible AChE inhibition at 10.9% at 100 µg/mL [62].

2.3.3. Other Polyketides

Compounds 2(4-hydroxyphenyl)acetic acid (240) and 2(2-hydroxyphenyl)acetic acid (241) (Figure 10) were identified from the endophyte Colletotrichum gloeosporioides. These compounds exhibited mild anti-AChE activity at 200 μg via bioautography [26].

Orcinol (242) was obtained from Penicillium sp.1, an endophytic fungus from the leaves of Alibertia macrophylla (Rubiaceae). It exhibited moderate AChE inhibition [25].

One bioactive compound, sorokiniol (243), was isolated from fungal endophyte Bipolaris sorokiniana LK12. It exhibited significant AChE inhibition with an EC₅₀ of 3.402 ± 0.08 μg/mL [43].

The chemical assay for endophyte Botryosphaeria dothidea KJ-1 led to the qualification of altenusin (244) and 5′-methoxy-6-methylbiphenyl-3,4,3′-triol (245), which displayed obvious DPPH scavenger activities with an IC₅₀ of 17.6 ± 0.23 and 18.7 ± 0.18 μM, respectively [33].

Parahydroxybenzaldehyde (246) collected from Epicoccum nigrum associated with the fresh leaves of E. abyssinica Steud. Ex A. Rich., Fabaceae, exhibited significant ABTS and DPPH scavenging activities with an IC₅₀ of 38.43 ± 4.85 and 49.45 ± 6.52 µg/mL, respectively [30].

Phomopsol B (247) and 248 were identified in Phomopsis sp. Xy21. Compound 248 was composed of a pair of epimers of 3-(2,6-dihydroxyphenyl)-4-hydroxy-6-methyl-isobenzofuran-1(3H)-one at C-9 and possessed neuroprotection, improving cell viability by 96% for corticosterone-mediated PC12 cell damage at 40.0 μM, whereas 247 did not display any such activity within the scope of 5.0−40.0 μM [38].

Three new p-terphenyls, 6′-O-Odesmethylterphenyllin (249), 3-hydroxy-6′-O-desmethylterphenyllin (250), and 3″-deoxy-6′-O-desmethylcandidusin B (252), along with two known p-terphenyls, 3,3″-dihydroxy-6′-O-desmethylterphenyllin (251) and 6′-O-desmethylcandidusin B (253), were collected from Penicillium chermesinum (ZH4-E2) associated with Kandelia candel. Compounds 252 and 253 inhibited AChE with IC₅₀s of 7.8 and 5.2 μM, respectively. The other compounds did not exhibit AChE inhibition with an IC₅₀ beyond 100 μM [57].

Chemical research on the endophytic Chaetomium globosum isolated from the seeds of Panax notoginseng resulted in the identification of flavipin (254), epicoccone (255), 3-methoxyepicoccone (256), and epicoccolides A (257) and B (258). Compound 256 possessed anti-AChE activity with an inhibition ratio of 72.6% at 50 μM. Compound 258 displayed obvious inhibitory activity against AChE with an IC₅₀ of 5.55 μM. The AChE inhibition rates of 254, 255, and 257 were lower than 10% at 50 μM. The structure–activity relationship revealed that the key group for AChE inhibition was an oxygenic five-membered ring between 256 and 258 [17].

A new enalin analog, 7-hydroxy-2,4dimethyl-3(2H)-benzofuranone (263), together with five known compounds, including butyrolactone I (259), ulocladol diacetate (260), ulocladol triacetate (261), 2,5-diacetylphenol (262), and enalin [2,7-dihydroxy-2,4-dimethyl-3(2H)-benzofuranone] (264), were isolated from Microsphaeropsis olivacea related to Pilgerodendron uviferum (D. Don) Florin (“Cipres de las Guaitecas”). The IC₅₀s of 260–262 for AChE inhibition were 83, 37, and 89 µg/mL, respectively [52].

An intensive chemical assay for Corynespora cassiicola L36 from Lindenbergi philippensis (Cham.) resulted in the observation of corynesidones A (265) and B (266), corynether A (267), and a diaryl ether (268). Corynesidone B (266) showed scavenging DPPH activity with IC₅₀ = 22.4 µM [63].

An investigation of Penicillium citrinum from Bruguiera gymnorrhiza led to the identification of (Z)-7,40-dimethoxy-6-hydroxy-aurone-4-O-b-glucopyranoside (269) and (1S,3R,4S)-1-(40-hydroxyl-phenyl)-3,4-dihydro-3,4,5-trimethyl-1H-2-benzopyran-6,8-diol (270). Compound 269 showed stronger neuroprotection than did 270 with respect to MPP⁺-mediated PC12 cell damage. The mechanism of 269 involved improving cell viability and mitochondrial membrane potential, inhibiting caspase-3 and caspase-9 expression and reducing DNA fragment formation [64].

The isobenzofuranone isopestacin (271) was identified in the endophytic fungus Pestalotiopsis microspora isolated from Terminalia morobensis. Compound 271 exhibited potent scavenging OH activity at 0.22 mM [65].

Oosporein (272) identified in the endophyte Cochliobolus kusanoi from Nerium oleander L demonstrated a 50% scavenging DPPH capacity at 0.194 mM [66].

The careful chemical study of Sporothrix sp. (#4335) revealed the isolation of sporothrins A–C (273–275) and sporothrin C (276), 1-hydroxy 8-methoxy-naphthalene (277), and 1,8-dimethoxy-naphthalene (278) (Figure 11). Compound 253 showed potent AChE inhibition with IC₅₀ at 1.05 μM [67,68].

Three novel aromatic polyketide dimers, bialternacins B–D (279–281), were collected from Alternaria sp. interrelated with the stem of Maianthemum bifolium. Compound 281 alone exhibited anti-AChE capacity with IC₅₀ at 68.3 μM [56].

The investigation of Phomopsis sp. NXZ-05 related to the twigs of Camptotheca acuminata DECNE. (Nyssaceae). revealed seven compounds: 8-O-acetylmultiplolide A (282), 8-O-acetyl-5,6-dihydro-5,6-epoxymultiplolide A (283), 5,6-dihydro-5,6-epoxymultiplolide A (284), 3,4-deoxy-3,4-didehydromul-tiplolide A (285), (4E)-6,7,9-trihydroxydec-4-enoic acid (286), methyl (4E)-6,7,9-trihydroxydec-4-enoate (287), and multiplolide A (288) (Figure 12). The evaluation of AChE inhibition for 282–284 and 288 indicated that 282 possessed obvious anti-AChE activity with an IC₅₀ of 1.19 mg/mL, while the other compounds exhibited no apparent activity with an IC₅₀ beyond 10 mg/mL [69].

Detailed chemical research on Cladosporium cladosporioides MA-299, an endophytic fungus from the mangrove plant leaves of Bruguiera gymnorrhiza, contributed to the isolation of new compounds 5R-hydroxyrecifeiolide (289), 5S-hydroxyrecifeiolide (290), ent-cladospolide F (291), cladospolide G (292), and cladospolide H (293) together with known compounds iso-cladospolide B (294) and pandangolide 1 (295). Among them, 291 alone exhibited strong AChE inhibition with an IC₅₀ value of 40.26 µM [70].

A study on Aspergillus flavus cf-5 from the red alga Corallina officinalis revealed the isolation of the new compound (8E,12Z)-10,11-dihydroxyoctadeca-8,12-dienoic acid (296), which had a weak AChE inhibitory capacity with a rate of 10.3% at 100 µg/mL [71].

2′-Deoxyribolactone (297) and hexylitaconic acid (298) were identified from a new endophyte Curvularia sp., which was discovered on the stem bark of Rauwolfia macrophylla. The IC₅₀s of 297 and 298 for inhibiting AChE were 1.93 and 1.54 μM, respectively [72].

A chemical assay for Talaromyces aurantiacus demonstrated the separation of two new compounds: talaromycins A (299) and B (300). The IC₅₀ of 299 for AChE inhibition was 12.63 μM [73].

The compound E-G6-32 (301) was isolated from the endophyte Curvularia sp. G6-32 from the plant Sapindus saponaria L. It showed anti-DPPH and anti-ABTS activities with inhibitory rates of 22.5% and 62.7%, respectively [74].

The extensive investigation of Daldinia sp. TJ403-LS1 collected from Anoectochilus roxburghii led to the identification of five new acetylenic phenol derivatives, daldiniols A–E (302–305, 308); one new benzofuran derivative, daldiniol F (309); one new naphthol derivative, daldiniol G (310); and two known analogs, 4-hydroxy-3-(3-methylbut-3-en-l-ynyl)benzyl alcohol (306) and methoxy-3-(3-methylbut-3-enl-ynyl)benzyl alcohol (307). The IC₅₀s of 306, 307, 309, and 310 for anti-BChE activities were 6.93 ± 0.71, 16.00 ± 0.30, 23.33 ± 0.55, and 15.53 ± 0.39 μM, respectively [75].

Three new oxygenated cyclohexanoids, speciosins U–W (311–313), along with 4-hydroxy-3-(3′-methylbut-3′-en-1′-ynyl)-benzoic acid (314) and 4-hydroxy-3-prenyl-benzoic acid (315), were reported in the Saccharicola sp. of Eugenia jambolana. Compound 311 alone exhibited inhibition toward huAChE-ICER and eeAChE-ICER with IC₅₀s of 0.076 ± 0.01 and 0.0047 ± 0.0009 mg/mL, respectively [58].

A comprehensive assay for Alternaria alternate from the leaves of Psidium littorale Raddi resulted in the discovery of a new liphatic polyketone, alternin A (316), as well as the known compounds stemphyperylenol (317), 3(ζ)hydroxy-octadeca-4(E),6(Z)-dienoic acid (318), E-7,9-diene-11-methenyl palmitic acid (319), p-hydroxybenzonic acid (320), and benzoic acid (321) (Figure 13). Compound 316 exhibited a significant neuroprotective capacity against glutamate-induced PC12 cell death, with cell viabilities improving from 64.7 ± 4.9% to 72.3 ± 4.5% after treatment with 20, 40, and 80 μM [29].

Two unusual dimers, trematosphones A (322) and B (323), were separated from the endophyte Trematosphaeria terricola isolated from desert plant Artemisia desertorum. Compound 322 alone dispalyed neuroprotection for corticosterone-induced PC12 cell damage at 6.25 μM [76].

A study on Phyllosticta capitalensis from the leaves of Loropetalum chinense var. rubrum led to the isolation of the new compound guignardianone G (324), together with three known compounds: xenofuranone B (325), linoleic acid (326), and 2-hexenoic acid (327). Compound 326 showed potential neuroprotective activities toward glutamate-injured PC12 cells with an EC₅₀ of 33.9 μM. Compound 324 showed no neuroprotective activity at 40 μM, and 325 and 327 even exhibited weak cytotoxicity at 40 μM [77].

A phthalide glycerol ether (328) was found in Cochliobolus lunatus SCSIO41401. This compound displayed mild AChE inhibition with IC₅₀ at 2.5 ± 0.21 μM, while the IC₅₀ for the active control of huperzine A was 0.30 ± 0.06 μM [40].

Phomeketales A–F (329–334) (Figure 14) were separated from Phoma sp. YN02-P-3. Compound 331 alone exhibited moderate AChE inhibition with IC₅₀ at 40.0 μM [78].

Extensive research on Penicillium sp. sk14JW2P collected from the roots of Kandelia candel (L.) DRUCE revealed the existence of 13-hydroxypalitantin (335) and (+)-palitantin (336), which exhibited anti-AChE activities with IC₅₀ values of 12 ± 0.3 and 79 ± 2 nM, respectively, while the IC₅₀ for the positive control of huperzine A was 0.06 μM [79].

The intensive study of endophyte Aspergillus sp. xy02 from a Thai mangrove Xylocarpus moluccensis uncovered seven new compounds, including (7R,10S)-7,10-epoxysydonic acid (337), (7S,10S)-7,10-epoxysydonic acid (338), (7R,11S)-7,12-epoxysydonic acid (339), (7S,11S)-7,12-epoxysydonic acid (340), 7-deoxy-7,14-didehydro-12-hydroxysydonic acid (341), (Z)-7-deoxy-7,8-didehydro-12-hydro-xysydonic acid (342), and (E)-7-deoxy-7,8-didehydro-12-hydroxysydonic acid (343), as well as five known compounds: (+)-1-hydroxyboivinianic acid (344), engyodontiumone I (345), (+)-sydonic acid (346), (+)-hydroxysydonic acid (347), and (−)-(7S)-10-hy-droxysydonic acid (348). Compound 348 alone displayed a moderate scavenging DPPH capacity with an IC₅₀ of 72.1 μM [80].

Intensive chemical research on Phaeosphaeria sp. LF5 from the leaves of Huperzia serrata generated the identification of 3-(hydroxymethyl)-5-methylfuran-2(5H)-one (349), aspilactonols G–I (350–352), and E-∆²-anhydromevalonic acid (353). Compound 352 alone exhibited anti-AChE activity with IC₅₀ at 6.26 µM. The other compounds showed no activity at 100 µM [49].

Investigation into a co-culture of endophyte Epicoccum sp. YUD17002 and Armillaria sp. contributed to the discovery of armilliphatics A–C (354–356). The IC₅₀ value of compound 354 for anti-AChE activity was 23.85 μM. The other compounds were inactive against AChE at 50 μM [81].

A rare 1-oxaspiro chaetospirolactone (357), orsellide F (358), orsellide A (359), globosumone B (360), and globosumone C (361) were obtained from Chaetoium sp. NF00754. The IC₅₀ values for compounds 359 and 361 for anti-AChE activity were 7.34 and 7.67 µM, respectively [82].

2.4. Terpenoids

2.4.1. Sesquiterpenoids

Two new compounds, asperterpenols A (362) and B (363) (Figure 15), with a rare 5/8/6/6 tetracyclic ring skeleton, were separated from Aspergillus sp. 085242. Compounds 362 and 363 powerfully inhibited AChE with IC₅₀s of 2.3 and 3.0 μM, respectively. Neither compound inhibited BChE (IC₅₀ >100 μM) [83].

The new compound (1R,5R,6R,7R,10S)-1,6-dihroxyeudesm-4(15)-ene (364) was identified from Alternaria alternate interrelated with the leaves of Psidium littorale Raddi. This compound was inactive for neuroprotective activity toward glutamate-injured PC12 cells at 40 and 80 μM [29].

The extensive chemical investigation of endophyte Paecilomyces sp. TE-540 associated with the fresh leaves of Nicotiana tabacum L. led to the identification of two new cadinane-type sesquiterpenes, paecilacadinols A (365) and B (366), and two new drimane-type sesquiterpenes, ustusol D (367) and ustusol E (368), along with known compounds 12-hydroxyalbrassitriol (369), 2-hydroxyalbrassitriol (370), deoxyuvidin B (371), 3β,9α,11-trihydroxy-6-oxodrim-7-ene (372), 2α,11-dihydroxy-6-oxodrim-7-ene (373), and ustusol B (374). The AChE inhibition ratios of 365–374 were in the range of 17.56 ± 3.33 to 57.38 ± 4.51%. The IC₅₀s of 369 and 370 for anti-AChE activities were 43.02 ± 6.01 and 35.97 ± 2.12 μM, respectively. The binding sites of 369 to the AChE catalytic pocket were Trp84, Gly117, Ser122, and Tyr121 residues, while 370 lay on Asp72 and Ser122 residues [84].

A study on Pseudofusicoccum sp. J003 from the mangrove species Sonneratia apetala Buch.-Ham led to the separation of the new sesquiterpene, acorenone C (375), which exhibited moderate activity against AChE with a 23.34% inhibition ratio at 50 μM [85].

Comprehensive research on Nemania bipapillata (AT-05) from the marine red alga Asparagopsis taxiformis-Falkenbergia stage led to the discovery of (+)-(2R,4S,5R,8S) (376), (+)-(2R,4R,5R,8S)-4-deacetyl-5-hydroxy-botryenalol (377), (+)-(2R,4S,5R,8R)-4-deacetyl-botryenalol (378), (+)-(2R,4R,8R) (379), (+)-(2R,4S,8S)-(380), and 4β-acetoxy-9β,10β,15α-trihydroxyprobotrydial (381). Compounds 376–381 showed AChE and BChE inhibition with inhibitory ratios of 18.3% and 27.7%, and 3.2% and 7.3% at 100 μM, respectively [86].

Guaidiol (382) was identified in Xylaria sp. HNWSW-2. The inhibition rate of 382 against AChE was 12.9% at 50 µg/mL [46].

Nigrosirpexin A (383) was collected from a co-culture of Nigrospora oryzae and Irpex lacteus. This compound showed an AChE inhibitory capacity with a ratio of 35% at 50 µM [87].

A chemical assay for Colletotrichum gloeosporioides GT-7 from the healthy tissue of Uncaria rhynchophylla produced colletotrichine A (384), which inhibited AChE with IC₅₀ at 28 μg/mL [88].

A co-culture of Armillaria sp. and endophyte Epicoccum sp. generated five protoilludane-type sesquiterpenoids, epicoterpenes A−E (385–389), which were inactive for AChE inhibition at 50 μM [47].

The comprehensive chemical investigation of Phomopsis sp. TJ507A from Phyllanthus glaucus led to the identification of a 2,3-seco-protoilludane-type sesquiterpene, phomophyllin A (390); eight protoilludane-type sesquiterpenes, phomophyllins B−I (391–398); four illudalane-type sesquiterpenes, phomophyllins J−M (399/400, 401, and 402); and a botryane-type sesquiterpene, phomophyllin N (403). In addition, seven known sesquiterpenoids, granulone B (404), radulone B (405), 2-(2,2,4,6-tetramethylindan-5-yl)ethanol (406), pterosin Z (407), onitin (408), dehydrobotrydienol (409), and 7-hydroxy-10-oxodehydrodihydrobotrydial (410), were also isolated from this fungus. This represents the first natural product of 390 with an irregular 2,3-seco-protoilludane skeleton. Compounds 390–396, 398, 405, 408, and 410 inhibited BACE1 within the range of 19.4% to 43.8% at 40 μM [89].

The fungus Colletotrichum gloeosporioides GT-7 generated the compound colletotrichine B (411), which inhibited AChE with IC₅₀ at 38.0 ± 2.67 μg/mL [90].

A chemical assay for Colletotrichum sp. SCSIO KcB3-2 from Kandelia candel produced a new polychiral bisabolane sesquiterpene of bisabolanoic acid A (412), which exhibited mild AChE inhibition with an IC₅₀ of 2.2 μM, whereas the IC₅₀ for the positive control of huperzine A was 0.30 ± 0.06 μM [91].

2.4.2. Meroterpenoids

Extensive research on Penicillium sp. SK5GW1L, a mangrove endophytic fungus from the leaves of Kandelia candel, resulted in the separation of two new α-pyrone meroterpenoids, arigsugacin I (413) and 3-epiarigsugacin E (416), together with seven known analogs: arigsugacin F (414), territrem B (415), arisugacin D (417), arisugacin B (418), territrem C (419), and terreulactone C (420) (Figure 16). The IC₅₀ values for all the isolates against AChE were 0.64, 0.37, 7.03, 38.23, 53.39, 3.03, 0.23, and 0.028 μM, respectively [92,93].

The investigation of Aspergillus terreus Thom, an endophytic fungus from Tripterygium wilfordii Hook. f. (Celastraceae), revealed six undescribed meroterpenoids, spiroterreusnoids A–F (421–426). The IC₅₀s of 421–426 for BACE1 and AChE inhibition ranged from 5.86 to 27.16 μM and from 22.18 to 32.51 μM, respectively [94].

A detailed study on Aspergillus 16-5c, a mangrove endophytic fungus identified from Sonneratia apetala, found one new meroterpenoid, 2-hydro-acetoxydehydroaustin (427), along with known analogs 11-acetoxyisoaustinone (428), isoaustinol (429), austin (430), austinol (431), acetoxydehydroaustin (432), dehydroaustin (433), dehydroaustinol (434), preaustinoid A2 (435), and 1,2-dihydro-acetoxydehydroaustin B (436). The IC₅₀s for AChE inhibition by compounds 429, 433, and 434 were 2.50, 0.40, and 3.00 µM, respectively [95].

2.4.3. Diterpenoids

Chemical research on Penicillium chrysogenum MT-12 collected from Huperzia serrata revealed the new compounds penicichrysogene A (437) and penicichryso-gene B (438) (Figure 17). Unfortunately, neither compound showed obvious AChE and BChE inhibition at 100 µM [96].

A study on the Aspergillus sp. YXf3 of Ginkgo biloba found an irregular C18 norditerpenoid, aspergiloid I (439), which did not exhibit antioxidant properties or AChE inhibition at 50 μg/mL [97].

2.5. Steroids

A new steroid, asporyergosterol (440), along with four known steroids, containing (22E,24R)-ergosta-4,6,8(14),22-tetraen-3-one (441), (22E,24R)-3β-hydroxyergosta-5,8,22-trien-7-one (442), (22E,24R)-ergosta-7,22-dien-3β,5α,6β-triol (443), and (22E,24R)-5α,8α-epidioxyergosta-6,22-dien-3β-ol (444) (Figure 18), were identified from culture extracts of Aspergillus oryzae associated with the marine red alga Heterosiphonia japonica. All the compounds exhibited a low capacity to modulate AChE with inhibitory rates from 0.4%–19.8% at 100 µg/mL [98].

The instentive investigation of Aspergillus flavus cf-5 from the marine red alga Corallina officinalis led to the separation of a new compound, 3β,4α-dihydroxy26-methoxyergosta-7,24(28)-dien-6-one (445), as well as four known isolates: episterol (446), (22E,24R)-ergosta7,22-dien-3β,5α,6α-triol (447), (22E,24R)-ergosta-5,22-dien-3β-ol (448), and (22E,24R)-ergosta-4,6,8(14),22-tetraen-3-one (441). Compound 445 displayed weak activity against AChE with an inhibition ratio of 5.5% at 100 µg/mL [71].

A study on Chaetomium sp. M453 associated with Huperzia serrata (Thunb. ex Murry) Trev produced the isolation of neocyclocitrinols E–G (449–451) and 3β-hydroxy-5,9-epoxy-(22E,24R)-ergosta-7,22-dien-6-one (452) as well as three known steroids (453–455) separated from the endophytic fungus Chaetomium sp. M453 associated with Huperzia serrata (Thunb. ex Murry) Trev. Compounds 451–452 were assayed for AChE inhibitory activities. Compound 452 alone showed weak AChE inhibitory activity at 50 μM [99].

Four known steroids, (3β,5α,6α, 22E)-3-hydroxy-5,6-epoxy7-one-8(14),22-dien-ergosta (456), 443, β-sitostenone (457), and β-sitosterol (458), and 448 were obtained from Chaetomium sp. YMF432 related to Huperzia serrata (Thunb. ex Murray) Trev. Compound 456 alone showed moderate AChE inhibition with an IC₅₀ of 67.8 ± 1.7 μM and an inhibitory rate of 58.8 % at 100 μg/mL [60].

An extensive study on Aspergillus terreus (No. GX7-3B) from a branch of Bruguiera gymnoihiza (Linn.) Savigny resulted in the separation of 3β,5α-dihydroxy-(22E,24R)-ergosta-7,22-dien-6-one (459), 3β,5α,14α-trihydroxy-(22E,24R)-ergosta-7, 22-dien-6-one (460), and NGA0187 (461). Compound 461 displayed remarkable anti-AChE activity with an IC₅₀ value of 1.89 μM [36].

Ergosterol (462) was identified from Curvularia sp. associated with Rauwolfia macsrophylla. The IC₅₀ of 462 for AChE inhibitory activity was 1.52 μM [72].

Two known steroids, 441 and (17R)-4-hydroxy-17-methylincisterol (463), were identified from Alternaria alternate related to the leaves of Psidium littorale Raddi. Compounds 441 and 463 were inactive for neuroprotective activity toward glutamate-injured PC12 cells at 40 and 80 μM, respectively [29].

Research on Colletotrichum sp. F168 from the plant Huperzia serrata Trev produced the compound ergosta-7,22-dien-5,9-epoxy-(22E,24R)-6-one-3-yl acetate (464), which showed a negligible AChE inhibitory activity of 18.2% at 100 µg/mL [62].

The investigation of Talaromyces sp. SCNU-F0041 from the fresh leaves of Kandelia produced cyclosecosteroid A (465), ergosterol (462), (22E,24R)-5α,8α-epidioxyergosta-6,22-dien-3β-ol (466), and cerevisterol (443). The IC₅₀ of compound 465 for inhibiting AChE was 46 μM [100].

Brassicasterol (448), 5,6-epoxyergosterol (454), citreoanthrasteroid A (467), demethylincisterol A (463), and chaxine C (468) were identified in Phyllosticta capitalensis derived from the leaves of Loropetalum chinense var. rubrum. Compound 467 alone exhibited neuroprotection with an EC₅₀ of 24.2 μM for glutamate-mediated PC12 cell injury [77].

3. Conclusions

Endophytic fungi are significant treasured natural products that provide numerous bioactive compounds for the research of new drugs. According to the statistical results (Tables S1–S5, Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17 and Figure 18), 468 metabolites with anti-AD-related activities and diverse structural features were identified in this study. These isolated natural products from endophytes possessed diverse structural features and included alkaloids (135, 28.8%), peptides (9, 1.9%), polyketides (217, 46.4%), terpenoids (78, 16.7%), and steroids (29, 6.2%) (Figure 19). Among these compounds, polyketides were the most common, followed by alkaloids, terpenoids, and steroids. A total of 468 compounds were isolated from 83 endophytes, which were assigned to 2 phyla, 5 classes, and 35 genera. Taxonomically, nearly all the strains belonged to the phyla Ascomycotina (98.8%), including the classes Eurotinomycetes (36.1%), Sordariomycetes (37.3%), Dothideomycetes (22.9%), and Leotiomycetes (2.4%), while only Agaricomycetes belonged to the phylum Dasidiomycota (1.2%) (Figure 20). Some genera contained two or more species of endophytes that possess promising bioactive anti-Alzheimer’s components, including Aspergillus (13), Penicillium (11), Colletotrichum (9), Phomopsis (5), Talaromyces (4), Chaetomium (4), Alternaria (2), Epicoccum (2), Cochliobolus (2), and Curvularia (2) (Figure 21). Around 27.5% of the compounds were separated from the genera Aspergillus and Penicillium, accounting for 72 and 58 compounds, respectively.

Based on the analyzed data, the biological activity of these compounds was determined, mainly focusing on their anti-AChE, anti-BChE, antioxidant, and neurotrophic activities. Some of the compounds exhibited micromolar to nanomolar biological activities, such as chaetoglobosin F (17) and isochaetoglobosin D (23), which showed strong H₂O₂-induced PC12 cell damage-inhibiting activities with EC₅₀s of 0.003 ± 0.0003 and 0.009 ± 0.001 μM, respectively. Huptremules C and D (118, 119) showed stronger AChE-inhibiting activities, with IC₅₀s of 0.11 ± 0.01 and 0.06 ± 0.00 μM, respectively, than hupA (IC₅₀ = 0.54 μM). Hence, they represent valuable compounds for developing anti-AD agents. Notably, structural changes to these compounds directly affect their bioactivities. Synthesis and structural modifications for bioactive metabolites are necessary to prepare more effective analogs. This review confirmed the significance of endophytes in the generation of abundant metabolic products with anti-AD activities. In the future, with the addition of further in-depth research on endophytic fungal metabolites, more biologically active chemical resources will become available to medicinal chemists and biologists for anti-AD drug research.