The incredible story of ophiobolin A and sphaeropsidin A: two fungal terpenes from wilt-inducing phytotoxins to promising anticancer compounds

Covering: 2000 to 2023This review presents the exceptional story of ophiobolin A (OphA) and sphaeropsidin A (SphA), a sesterterpene and a diterpene, respectively, which were initially isolated as fungal phytotoxins and subsequently shown to possess other interesting biological activities, including promising anticancer activities. Ophiobolin A is a phytotoxin produced by different fungal pathogens, all belonging to the Bipolaris genus. Initially, it was only known as a very dangerous phytotoxin produced by fungi attacking essential cereals, such as rice and barley. However, extensive and interesting studies were carried out to define its original carbon skeleton, which is characterized by a typical 5 : 8 : 5 ring system and shared with fusicoccins and cotylenins, and its phytotoxic activity on host and non-host plants. The biosynthesis of OphA was also defined by describing the different steps starting from mevalonate and through the rearrangement of the acyclic C-25 precursor lead the toxin is obtained. OphA was also produced as a bioherbicide from Drechslera gigantea and proposed for the biocontrol of the widespread and dangerous weed Digitaria sanguinaria. To date, more than sixty ophiobolins have been isolated from different fungi and their biological activities and structure-activity relationship investigated, which were also described using their hemisynthetic derivatives. In the last two decades, thorough studies have been performed on the potential anticancer activity of OphA and its original mode of action, attracting great interest from scientists. Sphaeropsidin A has a similar story. It was isolated as the main phytotoxin from Diplodia cupressi, the causal agent of Italian cypress canker disease, resulting in the loss of millions of plants in a few years in the Mediterranean basin. The damage to the forest, environment and ornamental heritage are noteworthy and economic losses are also suffered by tree nurseries and the wood industry. Six natural analogues of SphA were isolated and several interesting hemisynthetic derivatives were prepared to study its structure-activity relationship. Surprisingly, sphaeropsidin A showed other interesting biological activities, including antibiotic, antifungal, and antiviral. In the last decade, extensive studies have focused on the anticancer activity and original mode of action of SphA. Furthermore, specific hemisynthetic studies enable the preparation of derivatives of SphA, preserving its chromophore, which showed a noteworthy increase in anticancer activity. It has been demonstrated that ophiobolin A and sphaeropsidin A are promising natural products showing potent activity against some malignant cancers, such as brain glioblastoma and different melanomas.

The effect of protein kinase C activator and nitric oxide donor on oocyte activation and cortical granule exocytosis in porcine eggs

Nitric oxide (NO) and protein kinase C (PKC) are involved in the activation of mammalian oocytes, although their role in the exit from the metaphase II stage and cortical granule (CG) exocytosis is still not fully understood. The aim of this study was to verify whether the NO-donor together with specific PKC-activators induce the complete activation of porcine oocytes assessed as meiosis resumption and a cortical reaction. Pig maturated oocytes were treated with the NO-donor S-nitroso-N-acetylpenicillamine (SNAP, 2 mM) or PKC-activators such as phorbol-12-myristate-13-acetate (PMA, 100 nM), 1-oleoyl-2-acetyl-sn-glycerol (OAG, 400 μM) and l-α-phosphatidylinositol-3,4,5-trisphosphate dipalmitoyl heptaammonium salt (DPAM, 2 μM). To study the combined effect of NO-donor and PKC-activators, aliquots of oocytes were also incubated with SNAP (0.5 mM) together with PKC-activators at the same concentration as above (SNAP-DPAM, SNAP-OAG and SNAP-PMA groups). After in vitro maturation, an aliquot of oocytes was placed in a fresh medium without NO-donor or PKC-activators (Control group). Another aliquot of oocytes was activated by calcium ionophore A23187 (25 μM, 5 min). The results showed that 0% of the control oocytes reassumed meiosis. However, both the PKC-activators (DPAM 44.0 ± 10.0%, OAG 63.3 ± 1.0% and PMA 45.0 ± 16.5%) as well as the NO-donor alone (48.7 ± 21.0%) significantly induced exit from MII. Interestingly, the combination of PKC-activators and SNAP mainly restrained to the meiosis resumption (SNAP-OAG 0, SNAP-DPAM 17.4 ± 2.5% and SNAP-PMA 38.4 ± 8.5%). Control oocytes did not show a cortical reaction and the area occupied by CG reached 25.9 ± 1.7%, whereas CGs were partially released after Ca2+ ionophore treatment (13.0 ± 3.2%). Treatment with PKC-activators induced a cortical reaction compared with the control group (8.6 ± 2.5, 6.7 ± 1.9 and 0.7 ± 0.4%, respectively, for DPAM, OAG and PMA groups). However, treatment with the NO-donor alone (SNAP group 17.2 ± 2.2%) or combined with any PKC-activator prevented cortical reaction (SNAP-DPAM 20.7 ± 2.6%, SNAP-OAG 16.7 ± 2.9% or SNAP-PMA 20.0 ± 2.4%). Besides, meiosis resumption was not always accompanied by a cortical reaction, indicating that these two activation events are independent. In conclusion, PKC-activators alone induce CG exocytosis to the same degree as calcium ionophore. However, an NO-donor alone or combined with PKC-activators is not able to induce a cortical reaction in pig oocytes.

Ultrastructural localisation of calcium deposits in pig ovarian follicles

Calcium intracellular signaling regulates many intracellular events including oocyte maturation. This signaling is strongly dependent on the influx of calcium ions from extracellular spaces and on the state of intracellular calcium stores. In this study, intracellular calcium deposits were detected in follicle-enclosed pig oocytes using the combined oxalate-pyroantimonate method. These deposits were observed in the nucleus, the mitochondria, the cytoplasm, and on the surface of lipid droplets. The amount of calcium deposits was expressed as a percentage of the area of the respective cellular compartment, which is covered with calcium deposits on ultrathin sections. The distribution of calcium deposits in oocytes changed during folliculogenesis. The amount of calcium deposits in nuclei (1.11% of the area of oocyte nuclei) and cytoplasm (1.02%) in oocytes from secondary and early antral follicles (0.90% nuclei; 0.99% cytoplasm) is significantly lower (P < 0.05) than the amount of calcium deposits in these compartments in oocytes from primary follicles (2.51% nuclei; 2.34% cytoplasm) or antral follicles with growing oocyte (2.91% nuclei; 2.21% cytoplasm). The amount of calcium deposits in mitochondria of oocytes from primary follicles (1.27%) or antral follicles with growing oocyte (1.14%) is significantly lower (P < 0.05) than in the nucleus (2.51% in oocytes from primary follicles; 2.91% in growing oocytes from antral follicles) or cytoplasm (2.34% in oocytes from primary follicles; 2.21% in growing oocytes from antral follicles). The amount of calcium deposits in the cytoplasm of fully-grown oocytes (1.46%) dropped to levels significantly lower (P < 0.05) than those observed in the oocyte nucleus (2.29%). On the basis of these data, we can conclude that the population of follicles on pig ovaries differs in the distribution and concentration of calcium deposits in oocytes, and these changes may be involved in the regulation of the meiotic competence of oocytes.

Activation of pig oocytes using nitric oxide donors

Nitric oxide (NO) plays an important role in intracellular signaling, but its role during the activation of mammalian oocytes is little understood. In our study, in vitro matured pig oocytes were cultured with NO-donors-S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitropruside (SNP). These treatments were able to induce parthenogenetic activation of pig oocytes matured in vitro. The specificity of this effect was confirmed by the activation of oocytes by exogenous endothelial nitric oxide synthase (eNOS) microinjected in the oocyte with its activator calmodulin. Relatively long exposure (10 hr) is needed for activation of pig oocytes with 2.0 mM SNAP. An active NOS is necessary for the NO-dependent activation of pig oocytes because NOS inhibitors L-NMMA or L-NAME are able to inhibit activation of oocytes with NO-donor SNAP. On the basis of our data, we conclude that the NO-dependent activating stimulus seems inadequate because it did not induce the exocytosis of cortical granules. Also, the cleavage of parthenogenetic embryos was very low, and embryos did not develop beyond the stage of eight blastomeres.

Ultrastructural localisation of calcium deposits in the mouse ovary

Follicle-enclosed mouse oocytes contain numerous calcium deposits. The ultrastructural distribution of calcium deposits in the nuclei, mitochondria and cytoplasm of mouse oocytes and granulosa cells of primary, secondary and antral follicles was examined using the combined oxalate–pyroantimonate method. The mitochondria of oocytes from all types of follicles had the highest levels of calcium deposits of all oocyte compartments, with the exception of primary follicles, in which oocyte nuclei contained the same level of calcium deposits as the mitochondria. Calcium deposits in the cytoplasm of oocytes from primary follicles were significantly lower than those in the cytoplasm of oocytes from secondary and antral follicles. Calcium deposits in the cytoplasm of granulosa cells were significantly lower than calcium deposits in the mitochondria of granulosa cells and this difference persisted throughout all categories of follicles. Calcium deposits in the nuclei of granulosa cells did not differ from levels in the mitochondria in primary and secondary follicles. In contrast, the nuclei of granulosa cells from antral follicles had lower levels of calcium deposits than the mitochondria. The differences observed in calcium deposits in various cellular compartments in oocytes and granulosa cells in the follicles of ovaries of adult mice can be attributed to their acquisition of meiotic competence and follicular development.